System and method for dual-chamber pacing

ABSTRACT

A medical device system including an pacemaker implantable in a chamber of a patient&#39;s heart is configured to sense near field events from a cardiac electrical signal, establish a lower rate interval to control a rate of delivery of pacing pulses and schedule a first pacing pulse by starting a pacing escape interval set equal to the lower rate interval. The pacemaker withholds the scheduled pacing pulse in response to sensing a near-field event during the pacing escape interval and schedules a next pacing pulse to be delivered at the lower rate interval from a time that the pacing escape interval is scheduled to expire.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Nos.62/047,418, filed on Sep. 8, 2014 and 62/068,377, filed on Oct. 24,2014. The disclosure of the above applications are incorporated hereinby reference in its entirety.

TECHNICAL FIELD

The disclosure relates to an implantable medical device system andassociated method for controlling intracardiac pacemakers to delivercoordinated dual chamber pacing to a patient's heart.

BACKGROUND

Implantable cardiac pacemakers are often placed in a subcutaneous pocketand coupled to one or more transvenous medical electrical leads carryingpacing and sensing electrodes positioned in the heart. A cardiacpacemaker implanted subcutaneously may be a single chamber pacemakercoupled to one medical lead for positioning electrodes in one heartchamber, atrial or ventricular, or a dual chamber pacemaker coupled totwo leads for positioning electrodes in both an atrial and a ventricularchamber. Multi-chamber pacemakers are also available that may be coupledto three leads, for example, for positioning electrodes for pacing andsensing in one atrial chamber and both the right and left ventricles.

Intracardiac pacemakers have recently been introduced that are whollyimplantable within a ventricular chamber of a patient's heart fordelivering ventricular pacing pulses. Such a pacemaker may sense R-wavesignals attendant to intrinsic ventricular depolarizations and deliverventricular pacing pulses in the absence of sensed R-waves. While singlechamber ventricular pacing may adequately address some patientconditions, other conditions may require atrial and ventricular pacing,commonly referred to a dual chamber pacing, in order to maintain aregular heart rhythm.

SUMMARY

In general, the disclosure is directed to implantable medical device(IMD) systems that include an atrial intracardiac pacemaker andtechniques for controlling pacing pulses delivered by the atrialintracardiac pacemaker to deliver coordinated dual chamber pacing. TheIMD system may include a ventricular intracardiac pacemaker for pacingin a ventricular chamber. An intracardiac atrial pacemaker operating inaccordance with the techniques disclosed herein controls atrial pacingescape intervals in response to atrial events and far-field ventricularevents that are sensed by the atrial intracardiac pacemaker. The atrialintracardiac pacemaker delivers atrial pacing pulses that arecoordinated with ventricular events without requiring sensing of atrialevents by a ventricular intracardiac pacemaker when present.

In one example, the disclosure provides an implantable medical device(IMD) system comprising a pacemaker implantable in a chamber of a heartof a patient. The pacemaker comprises a sensing module configured toreceive a cardiac electrical signal and sense near field events in theheart chamber from the cardiac electrical signal, a pulse generatorconfigured to generate and deliver pacing pulses to the heart chambervia a pair of electrodes, and a control module coupled to the sensingmodule and the pulse generator. The control module is configured toestablish a lower rate interval to control a rate of delivery of thepacing pulses, schedule a first pacing pulse to be delivered by thepulse generator by starting a pacing escape interval equal to the lowerrate interval, withhold the scheduled pacing pulse in response to thesensing module sensing a near-field event during the pacing escapeinterval, and schedule a next pacing pulse to be delivered at the lowerrate interval from a time that the pacing escape interval is scheduledto expire.

In another example, the disclosure provides a method comprising sensingnear field events in a chamber of a patient's heart from a cardiacelectrical signal received by a sensing module of a pacemakerimplantable in the heart chamber, establishing by the pacemaker a lowerrate interval to control a rate of delivery of pacing pulses, schedulinga first pacing pulse by starting a pacing escape interval set equal tothe lower rate interval, withholding the scheduled pacing pulse inresponse to sensing a near-field event during the pacing escapeinterval; and scheduling a next pacing pulse to be delivered at thelower rate interval from a time that the pacing escape interval isscheduled to expire.

In yet another example, the disclosure provides a non-transitory,computer-readable storage medium comprising a set of instructions which,when executed by a control module of a pacemaker implantable in achamber of a patient's heart, cause the pacemaker to sense near fieldevents from a cardiac electrical signal, establish a lower rate intervalto control a rate of delivery of pacing pulses, schedule a first pacingpulse by starting a pacing escape interval set equal to the lower rateinterval, withholding the scheduled pacing pulse in response to sensinga near-field event during the pacing escape interval; and scheduling anext pacing pulse to be delivered at the lower rate interval from a timethat the pacing escape interval is scheduled to expire.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the apparatus and methods described indetail within the accompanying drawings and description below. Furtherdetails of one or more examples are set forth in the accompanyingdrawings and the description below

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an intracardiac pacingsystem that may be used to sense cardiac electrical signals and providetherapy to a patient's heart.

FIG. 2A is a conceptual diagram of an intracardiac pacemaker.

FIGS. 2B and 2C are conceptual diagrams of alternative embodiments of anintracardiac pacemaker.

FIG. 3 is a functional block diagram of an example configuration of theintracardiac pacemaker shown in FIG. 2A.

FIG. 4 is a flow chart of a method for controlling atrial pacing pulsedelivery by an atrial intracardiac pacemaker.

FIGS. 5A through 5C are timing diagrams illustrating methods forcontrolling atrial pacing pulse delivery by an atrial intracardiacpacemaker in the presence of atrial sense events.

FIG. 6 is a timing diagram illustrating methods for controlling atrialpacing pulse delivery by an atrial intracardiac pacemaker in thepresence of ventricular sense events.

FIG. 7 is a flow chart of a method for controlling an atrialintracardiac pacemaker to provide coordinated dual chamber pacing.

FIGS. 8 through 11 are timing diagrams illustrating operations shown inthe flow chart of FIG. 7 for controlling atrial pacing pulse delivery byan atrial intracardiac pacemaker for delivering coordinated atrial andventricular pacing using separate intracardiac pacemakers.

FIG. 12 is a flow chart of a method for controlling ventricular pacingpulses by a ventricular intracardiac pacemaker according to one example.

DETAILED DESCRIPTION

An implantable medical device (IMD) system is disclosed herein thatincludes an intracardiac pacemaker configured to be implanted wholly ina chamber of the patient's heart. In various examples, the IMD systemincludes an atrial intracardiac pacemaker and a ventricular intracardiacpacemaker that do not require transvenous leads but are enabled toprovide coordinated atrial and ventricular pacing without wireless orwired communication signals between the two intracardiac pacemakers. Anatrial intracardiac pacemaker included in the system includes a controlmodule that monitors ventricular events and controls atrial pacing pulsedelivery based on sensed ventricular events (or lack thereof) to promoteatrial-ventricular synchrony.

In past practice, a dual chamber pacemaker positioned in an implantpocket and coupled to transvenous atrial and ventricular leads may beprogrammed to deliver only atrial pacing (AAI(R)), only ventricularpacing (VVI(R)) or both (DDD(R)) according to patient need. The dualchamber pacemaker is able to control the delivery of pacing pulses inboth atrial and ventricular chambers because the pacemaker will receivecardiac event signals from both atrial and ventricular chambers viacorrespondingly placed sensing electrodes and control when a pacingpulse is delivered in both chambers relative to the sensed events usingthe electrodes positioned in both chambers. In other words, the dualchamber pacemaker “knows” when both sensed and paced events haveoccurred in both atrial and ventricular pacing channels since allsensing and pacing control is happening in the one device, i.e., thedual chamber pacemaker.

Intracardiac pacemakers adapted to be implanted wholly within a heartchamber eliminate the need for transvenous, intracardiac leads.Complications due to infection associated with a lead extending from asubcutaneous pacemaker pocket transvenously into the heart can beeliminated. Other complications such as “Twiddler's syndrome”, leadfracture or poor connection of the lead to the pacemaker are eliminatedin the use of an intracardiac pacemaker.

An intracardiac pacemaker can operate in a single chamber mode, e.g.,AAI or VVI, by delivering pacing pulses and inhibiting pacing when anintrinsic event is sensed in the chamber that the pacemaker is implantedin. While some patients may require only single chamber pacing andsensing, patients having AV conduction defects may require a pacingsystem capable of a coordinated dual chamber pacing mode to providepacing in the ventricle that is coordinated with atrial events. Anatrial intracardiac pacemaker and associated techniques are disclosedherein which provide ventricular-synchronized atrial pacing to promoteatrial pacing at a target atrioventricular (AV) interval relative toventricular events, which may include ventricular pacing pulses beingdelivered by an intracardiac ventricular pacemaker operatingindependently of the atrial intracardiac pacemaker. Maintaining a targetAV interval is important for maintaining proper filling of theventricles and promoting optimal cardiac hemodynamic function inpatient's having intrinsic AV conduction defects.

FIG. 1 is a conceptual diagram illustrating an intracardiac pacingsystem 10 that may be used to sense cardiac electrical signals andprovide therapy to a patient's heart 8. IMD system 10 includes a rightatrial (RA) intracardiac pacemaker 12 and a right ventricular (RV)intracardiac pacemaker 14. Pacemakers 12 and 14 are transcatheterintracardiac pacemakers adapted for implantation wholly within a heartchamber, e.g., wholly within the RV, wholly within the left ventricle(LV), wholly within the RA or wholly within the left atrium (LA) ofheart 8. In the example of FIG. 1, pacemaker 12 is positioned along anendocardial wall of the RA, e.g., along the RA lateral wall or RAseptum. Pacemaker 14 is positioned along an endocardial wall of the RV,e.g., near the RV apex. The techniques disclosed herein, however, arenot limited to the pacemaker locations shown in the example of FIG. 1and other positions and relative locations from each other are possible.In some examples, a RA intracardiac pacemaker 12 and a LV intracardiacpacemaker are implanted for delivering coordinated atrial andventricular pacing using the techniques disclosed herein.

Pacemakers 12 and 14 are reduced in size and may be generallycylindrical in shape to enable transvenous implantation via a deliverycatheter. In other examples, pacemakers 12 and 14 may be positioned atany other location inside or outside heart 8, including epicardiallocations. For example, pacemaker 12 may be positioned outside or withinthe right atrium or left atrium to provide respective right atrial orleft atrial pacing. Pacemaker 14 may be positioned outside or within theright ventricle or left ventricle to provide respective rightventricular or left ventricular pacing.

Pacemakers 12 and 14 are each capable of producing electricalstimulation pulses, i.e., pacing pulses, delivered to heart 8 via one ormore electrodes on the outer housing of the pacemaker. RA pacemaker 12is configured to sense an intracardiac electrogram (EGM) signal in theRA using the housing based electrodes and deliver RA pacing pulses. RVpacemaker 14 is configured to sense an EGM signal in the RV using one ormore housing based electrodes and deliver RV pacing pulses.

The RA pacemaker 12 and the RV pacemaker 14 are configured to controlthe delivery of pacing pulses to the respective atrial and ventricularchambers in a manner that promotes maintaining a target AV intervalbetween atrial events (e.g., P-waves or atrial pacing pulses) andventricular events (e.g., R-waves or ventricular pacing pulses). Atarget AV interval may be a programmed value selected by a clinician. Atarget AV interval may be identified as being hemodynamically optimalfor a given patient based on clinical testing or assessments. Each ofthe RA pacemaker 12 and RV pacemaker 14 include a control module thatcontrols functions performed by the respective pacemaker. The controlmodule of the RA pacemaker 12 is configured to automatically adjustatrial pacing escape intervals to increase the likelihood that an atrialpacing pulse is delivered at a target AV interval prior to an RV event.The control module of the RV pacemaker 14 may be configured to controlventricular pacing pulses to minimize ventricular pacing when intrinsicAV conduction is intact, but may be operating independently of RApacemaker 12 in that RV pacemaker 14 may not be configured to senseatrial events. Both the RA pacemaker control module and the RV pacemakercontrol module may be configured to adjust pacing timing intervals basedon a sensor signal correlated to the metabolic demand of the patient toprovide rate responsive pacing.

Pacemaker 12 and 14 are each capable of bidirectional wirelesscommunication with an external device 20. External device 20 may be aprogrammer used by a clinician or other user in a medical facility, ahome monitor located in a patient's home, or a handheld device. Aspectsof external device 20 may generally correspond to the externalprogramming/monitoring unit disclosed in U.S. Pat. No. 5,507,782(Kieval, et al.), hereby incorporated herein by reference in itsentirety.

External device 20 may be configured to establish a wireless radiofrequency (RF) communication link 22 with RA pacemaker 12 and wirelessRF communication link 24 with RV pacemaker 14 using a communicationprotocol that appropriately addresses the targeted pacemaker 12 or 14.An example RF telemetry communication system that may be implemented insystem 10 is generally disclosed in U.S. Pat. No. 5,683,432 (Goedeke, etal.), hereby incorporated herein by reference in its entirety.

External device 20 may be used for retrieving data from pacemakers 12and 14 and for sending data to pacemakers 12 and 14. Examples ofretrieved data include physiological signals such as RA or RV EGMsignals, therapy delivery data such as a history of pacing frequency,results of device diagnostic testing, current operating controlparameters or other data stored by the pacemaker. Data sent topacemakers 12 and 14 may include programmable control parameters used bythe pacemakers 12 and 14 to control sensing and pacing functions.

RA pacemaker 12 and RV pacemaker 14 may or may not be configured tocommunicate directly with each other. For example, neither RA pacemaker12 nor RV pacemaker 14 may be configured to initiate an RF communicationsession with the other device. Both pacemakers 12, 14 may be configuredto periodically “listen” for a valid “wake up” telemetry signal fromexternal device 20 and power up its own telemetry module to establish acommunication link 22 or 24 in response to a valid RF telemetry signal(or go back to “sleep” if no valid telemetry signal is received).However, pacemakers 12 and 14 may not be configured to transmit a “wakeup” signal to the other pacemaker to initiate a communication session.In other examples, the pacemakers 12 and 14 may be configured tocommunicate with each other, but, in order to conserve battery life ofthe intracardiac pacemakers, telemetry communication may be minimized.As such, communication does not occur on a beat-by-beat basis betweenthe RA pacemaker 12 and RV pacemaker 14 for communicating when the otherpacemaker is sensing cardiac events or when it is delivering pacingpulses.

In accordance with techniques disclosed herein, RA pacemaker 12 isconfigured to sense far-field (FF) ventricular events from the RA EGMsignal. FF ventricular events may include ventricular pacing pulsesdelivered by RV pacemaker 14 and/or R-waves, e.g., associated withpacing evoked responses, intrinsically conducted ventriculardepolarizations, and premature ventricular contractions. In someexamples, RA pacemaker 12 includes a sensing module configured to senseFF ventricular events from heart sounds using an acoustical sensor.

RV pacemaker 14 may or may not be configured to sense far-field P-wavesand FF atrial pacing pulses from the RV EGM signal. In some examples, RVpacemaker 14 is not configured to sense atrial events. In otherexamples, RV pacemaker 14 is configured to sense atrial events and mayinclude a conductor extending from the pacemaker housing to increasesensing electrode distance to improve FF atrial event sensing.

FIG. 2A is a conceptual diagram of an intracardiac pacemaker 100 thatmay correspond to RA pacemaker 12 or RV pacemaker 14 shown in FIG. 1.Pacemaker 100 includes electrodes 162 and 164 spaced apart along thehousing 150 of pacemaker 100 for sensing cardiac EGM signals anddelivering pacing pulses. Electrode 164 is shown as a tip electrodeextending from a distal end 102 of pacemaker 100, and electrode 162 isshown as a ring electrode along a mid-portion of housing 150, forexample adjacent proximal end 104. Distal end 102 is referred to as“distal” in that it is expected to be the leading end as it advancedthrough a delivery tool, such as a catheter, and placed against a targetpacing site.

Electrodes 162 and 164 form an anode and cathode pair for bipolarcardiac pacing and sensing. Electrodes 162 and 164 may be positioned onor as near as possible to respective proximal and distal ends 104 and102 to increase the inter-electrode spacing between electrodes 162 and164. Relatively greater inter-electrode spacing will increase thelikelihood of sensing FF signals that may be used by the pacemaker 100for sensing events in another heart chamber. For example, an increasedinter-electrode spacing between electrodes 162 and 164 when pacemaker100 is used as an RV pacemaker 14 may improve reliable sensing of FFP-waves.

In alternative embodiments, pacemaker 100 may include two or more ringelectrodes, two tip electrodes, and/or other types of electrodes exposedalong pacemaker housing 150 for delivering electrical stimulation toheart 8 and sensing EGM signals. Electrodes 162 and 164 may be, withoutlimitation, titanium, platinum, iridium or alloys thereof and mayinclude a low polarizing coating, such as titanium nitride, iridiumoxide, ruthenium oxide, platinum black among others. Electrodes 162 and164 may be positioned at locations along pacemaker 100 other than thelocations shown.

Housing 150 is formed from a biocompatible material, such as a stainlesssteel or titanium alloy. In some examples, the housing 150 may includean insulating coating. Examples of insulating coatings include parylene,urethane, PEEK, or polyimide among others. The entirety of the housing150 may be insulated, but only electrodes 162 and 164 uninsulated. Inother examples, the entirety of the housing 150 may function as anelectrode instead of providing a localized electrode such as electrode162. Alternatively, electrode 162 may be electrically isolated from theother portions of the housing 150.

The housing 150 includes a control electronics subassembly 152, whichhouses the electronics for sensing cardiac signals, producing pacingpulses and controlling therapy delivery and other functions of pacemaker100. Housing 150 further includes a battery subassembly 160, whichprovides power to the control electronics subassembly 152. Batterysubassembly 160 may include features of the batteries disclosed incommonly-assigned U.S. Pat. No. 8,433,409 (Johnson, et al.) and U.S.Pat. No. 8,541,131 (Lund, et al.), both of which are hereby incorporatedby reference herein in their entirety.

Pacemaker 100 may include a set of fixation tines 166 to securepacemaker 100 to patient tissue, e.g., by interacting with theventricular trabeculae. Fixation tines 166 are configured to anchorpacemaker 100 to position electrode 164 in operative proximity to atargeted tissue for delivering therapeutic electrical stimulationpulses. Numerous types of active and/or passive fixation members may beemployed for anchoring or stabilizing pacemaker 100 in an implantposition. Pacemaker 100 may include a set of active fixation tines asdisclosed in commonly-assigned, pre-grant publication U.S. 2012/0172892(Grubac, et al.), hereby incorporated herein by reference in itsentirety.

Pacemaker 100 may further include a delivery tool interface 158.Delivery tool interface 158 is located at the proximal end 104 ofpacemaker 100 and is configured to connect to a delivery device, such asa catheter, used to position pacemaker 100 at an implant location duringan implantation procedure, for example within a heart chamber.

A reduced size of pacemaker 100 enables implantation wholly within aheart chamber. In FIG. 1, RA pacemaker 12 and RV pacemaker 14 may havedifferent dimensions. For example, RA pacemaker 12 may be smaller involume than pacemaker 14, e.g., by reducing battery size, to accommodateimplantation in the smaller heart chamber. As such, it is recognizedthat pacemaker 100 may be adapted in size, shape, electrode location orother physical characteristics according to the heart chamber in whichit will be implanted.

FIG. 2B is a conceptual diagram of an alternative embodiment of anintracardiac pacemaker 110. Pacemaker 110 includes a housing 150,control assembly 152, battery assembly 160, fixation member 166 andelectrode 164 along a distal end 102, and may include a delivery toolinterface 158 along the proximal end 104 as described above inconjunction with FIG. 2A. Pacemaker 110 is shown to include an electrode162′ extending away from housing 150 along an extender 165. As such,instead of carrying a pair of electrodes along the housing 150, whichlimits the maximum possible inter-electrode spacing, an extender 165 maybe coupled to the housing 150 using necessary electrical feedthroughsfor positioning an electrode 162′ at an increased inter-electrodedistance from distal tip electrode 164.

For examples of an intracardiac pacemaker having increasedinter-electrode spacing between electrodes, reference is made tocommonly-assigned, pre-grant U.S. Publication No. 2013/0035748 (Bonner,et al.) and U.S. Patent Application Ser. No. 62/025,690, filed on Jul.17, 2014, both of which are incorporated herein by reference theirentirety.

FIG. 2C is a conceptual diagram of an alternative embodiment ofintracardiac pacemaker 120 having extender 165 coupled to the distal end102 of pacemaker housing 150 to extend distal electrode 164′ away fromelectrode 162 positioned along housing 150 near or at proximal end 104.Extender 165 shown in FIGS. 2B and 2C is an insulated electricalconductor that electrically couples electrode 162′ (FIG. 2B) orelectrode 164′ (FIG. 2C) to pacemaker circuitry via an electricalfeedthrough crossing housing 150. Pacemaker 120 having an insulated,electrically conductive extender 165 for increasing the inter-electrodespacing may correspond generally to the implantable device and flexibleconductor disclosed in the above incorporated U.S. Publication No.2013/0035748 (Bonner, et al.).

FIG. 3 is a functional block diagram of an example configuration ofpacemaker 100 shown in FIG. 2A. Pacemaker 100 includes a pulse generator202, a sensing module 204, a control module 206, memory 210, telemetrymodule 208 and a power source 214. As used herein, the term “module”refers to an application specific integrated circuit (ASIC), anelectronic circuit, a processor (shared, dedicated, or group) and memorythat execute one or more software or firmware programs, a combinationallogic circuit, or other suitable components that provide the describedfunctionality. Each of RA pacemaker 12 and RV pacemaker 14 will includesimilar modules as represented by the pacemaker 100 shown in FIG. 3;however it is understood that the modules are configured differently asneeded to perform the functionality of the separate RA and RV pacemakers12 and 14 as disclosed herein.

For example, when pacemaker 100 is a RA pacemaker 12, control module 206is configured to set various atrial pacing escape intervals used tocontrol delivery of atrial pacing pulses as disclosed herein. Whenpacemaker 100 is embodied as RV pacemaker 14, control module 206 isconfigured to set ventricular pacing escape intervals to controldelivery of RV pacing pulses according to techniques disclosed herein.Adaptations of the hardware, firmware or software of the various modulesof pacemaker 100 necessary to meet the described functionality of theintracardiac pacemakers positioned in different heart chambers asdisclosed herein is understood to be included in the various modules ofpacemaker 100 according to the intended implant location.

The functions attributed to pacemaker 100 herein may be embodied as oneor more processors, controllers, hardware, firmware, software, or anycombination thereof. Depiction of different features as specificcircuitry or modules is intended to highlight different functionalaspects and does not necessarily imply that such functions must berealized by separate hardware or software components or by anyparticular architecture. Rather, functionality associated with one ormore modules, processors, or circuits may be performed by separatehardware or software components, or integrated within common hardware orsoftware components. For example, pacing control operations performed bypacemaker 100 may be implemented in control module 206 executinginstructions stored in associated memory 210 and relying on input fromsensing module 204.

Pulse generator 202 generates electrical stimulation pulses that aredelivered to heart tissue via electrodes 162 and 164. Electrodes 162 and164 may be housing-based electrodes as shown in FIG. 2A, but one or bothelectrodes 162 and 164 may alternatively be carried by an insulated,electrical conductor extending away from the pacemaker housing asdescribed in conjunction with FIGS. 2B and 2C.

Pulse generator 202 may include one or more capacitors and a chargingcircuit to charge the capacitor(s) to a programmed pacing pulse voltage.At appropriate times, e.g., as controlled by a pacing escape intervaltimer included in a pace timing and control circuit in control module206, the capacitor is coupled to pacing electrodes 162 and 164 todischarge the capacitor voltage and thereby deliver the pacing pulse.Pacing circuitry generally disclosed in the above-incorporated U.S. Pat.No. 5,507,782 (Kieval, et al.) and in commonly assigned U.S. Pat. No.8,532,785 (Crutchfield, et al.), both of which patents are incorporatedherein by reference in their entirety, may be implemented in pacemaker100 for charging a pacing capacitor to a predetermined pacing pulseamplitude under the control of control module 206 and delivering apacing pulse.

Control module 206 controls pulse generator 202 to deliver a pacingpulse in response to expiration of a pacing escape interval according toprogrammed therapy control parameters stored in memory 210. The pacetiming and control circuit included in control module 206 includes anescape interval timer or counter that is set to various pacing escapeintervals used for controlling the timing of pacing pulses relative to apaced or sensed event. Upon expiration of a pacing escape interval, apacing pulse is delivered. If a cardiac event is sensed during thepacing timing interval by sensing module 204, the scheduled pacing pulsemay be inhibited, and the pacing escape interval may be reset to a newtime interval. Control of pacing escape intervals by control module 206are described below in conjunction with the various flow charts andtiming diagrams presented herein.

Sensing module 204 includes cardiac event detectors 222 and 224 forreceiving cardiac EGM signals developed across electrodes 162 and 164. Acardiac event is sensed by sensing module 204 when the EGM signalcrosses a sensing threshold of a cardiac event detector 222 or 224 insome examples. The sensing threshold may be an auto-adjusting sensingthreshold that may be initially set based on the amplitude of a sensedevent and decays at a predetermined decay rate thereafter. In responseto a sensing threshold crossing, sensing module 204 passes a sensedevent signal to control module 206.

Sensing module 204 may include a near-field (NF) event detector 222 anda far-field (FF) event detector 224. NF cardiac events are events thatoccur in the heart chamber where the electrodes 162 and 164 are located.FF cardiac events are events that occur in a different heart chamberthan the heart chamber where electrodes 162 and 164 are located.

The NF cardiac event detector 222 of RA pacemaker 12 may be programmedor configured to operate using a sensing threshold appropriate forsensing P-waves attendant to the depolarization of the atria. The NFcardiac event detector 222 of RV pacemaker 14 may be programmed orconfigured to operate using a sensing threshold appropriate for sensingR-waves attendant to the depolarization of the ventricles. NF cardiacevent detector 222 produces a sensed event signal provided to controlmodule 206 in response to sensing a NF event, i.e., a P-wave by RApacemaker 12 or an R-wave by RV pacemaker 14.

The terms “sensed cardiac events” or “sensed events” as used hereinrefer to events sensed by sensing module 204 in response to the EGMsignal crossing a sensing threshold, which may be an amplitudethreshold, a frequency threshold, a slew rate threshold, or anycombination thereof. Sensed cardiac events may include intrinsic eventsand evoked events. Evoked events include P-waves in the atria or R-wavesin the ventricle caused by a pacing pulse delivered in the respectiveheart chamber. Intrinsic events are events arising in the heart in theabsence of a pacing pulse delivered in the heart chamber in which theintrinsic event is sensed. Intrinsic events include intrinsic P-waves,such as sinus P-waves originating from the sino-atrial node of theheart, and intrinsic R-waves, such as sinus R-waves conducted throughthe heart's normal conduction pathway to the ventricles from the atriavia the atrioventricular node. Intrinsic events can also includenon-sinus intrinsic events, such as premature atrial contractions (PACs)or premature ventricular contractions (PVCs) that arise intrinsicallyfrom the heart but are ectopic in origin.

FF event detector 224 may be configured to sense FF ventricular eventswhen pacemaker 100 is embodied as RA pacemaker 12. A FF ventricularevent sensing threshold may be used by FF event detector 224 for sensingFF ventricular events. FF event detector 224 produces a FF sensed eventsignal that is passed to control module 206 in response to sensing a FFevent. FF ventricular events sensed by FF event detector 224 may includeventricular pacing pulses delivered by RV pacemaker 14 and/or R-waves,intrinsic or evoked. The FF event detector 224 may or may not beconfigured to discriminate between sensed FF ventricular events that arepacing pulses and sensed FF ventricular events that are R-waves.

As used herein, “FF ventricular events” may refer collectively to bothventricular pacing pulses and ventricular R-waves that are sensed by FFevent detector 224 in RA pacemaker 12, which may produce FF ventricularevent sense signals in response to the EGM signal meeting ventricularevent sensing criteria that may include sensing of both ventricularpacing pulses and R-waves non-discriminately. In other examples, the FFevent detector 224 may be configured to discriminately sense ventricularpacing pulse and/or ventricular R-waves and provide different senseevent signals to control module 206 in response to each.

In some examples, RV pacemaker 14 does not include a FF event detector224 configured to sense FF atrial events. In this case, RV pacemaker 14is configured for single chamber sensing of R-waves in the ventricle. FFP-waves are relatively small amplitude signals compared to NF R-wavesand may be difficult to distinguish from baseline noise on theventricular EGM signal. As described in conjunction with the flow chartsand timing diagrams disclosed herein, coordinated atrial and ventricularpacing may be provided by RA pacemaker 12 and RV pacemaker 14 withoutrequiring RV pacemaker 14 to sense FF atrial events.

In other examples, RV pacemaker may include a FF event detector 224configured to sense FF atrial pacing pulses and/or FF atrial P-waves,intrinsic or evoked. The inter-electrode spacing of sensing electrodes162 and 164 may be increased to enhance sensing of small amplitude FFP-waves by FF event detector 224, e.g., by using an extender as shown inFIG. 2B and FIG. 2C.

When available, FF atrial event signals produced by FF event detector224 in RV pacemaker 14 may be used by control module 206 of RV pacemaker14 to deliver atrial-synchronized ventricular pacing. FF atrial events,however, may be undersensed by RV pacemaker 14. Using the techniquesdisclosed herein, RA pacemaker 14 is configured to deliver atrial pacingpulses in a manner that maintains coordinated atrial and ventricularactivity even in the absence or loss of FF atrial event sensing by RVpacemaker 14.

Memory 210 may include computer-readable instructions that, whenexecuted by control module 206, cause control module 206 to performvarious functions attributed throughout this disclosure to pacemaker100. The computer-readable instructions may be encoded within memory210. Memory 210 may include any non-transitory, computer-readablestorage media including any volatile, non-volatile, magnetic, optical,or electrical media, such as a random access memory (RAM), read-onlymemory (ROM), non-volatile RAM (NVRAM), electrically-erasableprogrammable ROM (EEPROM), flash memory, or other digital media with thesole exception being a transitory propagating signal. Memory 210 maystore timing intervals, counters, or other data used by control module206 to control the delivery of pacing pulses by pulse generator 202,e.g., by setting a pacing escape interval timer included in controlmodule 206, according to the techniques disclosed herein.

Pacemaker 100 may further include one or more physiological sensors 212used for monitoring the patient. In some examples, physiological sensors212 include at least one physiological sensor producing a signalindicative of the metabolic demand of the patient. The signal indicativeof the patient's metabolic demand is used by control module 206 fordetermining a sensor indicated pacing rate to control a pacing rate thatmeets the patient's metabolic demand. For example, sensors 212 mayinclude an accelerometer for producing a patient activity signal passedto control module 206. An accelerometer included in sensors 212 may beembodied as a piezoelectric crystal for producing a signal correlated topatient body motion. The use of an accelerometer in an intracardiacdevice for obtaining a patient activity signal is generally disclosed inU.S. patent application Ser. No. 14/174,514 filed on Feb. 6, 2014(Nikolski, et al.), incorporated herein by reference in its entirety.

The accelerometer signal is used by the control module 206 to determinea sensor-indicated rate (SIR) used to establish a temporary lower rateinterval. The control module 206 sets the pacing escape interval basedon the established lower rate interval for controlling the pacing rateto meet the metabolic demand of the patient. RA pacemaker 12 mayinitially set the atrial pacing escape interval timer included incontrol module 206 to a lower rate interval corresponding to aprogrammed base pacing rate to provide bradycardia pacing. The lowerrate interval may be shortened from the base lower rate intervalautomatically to provide atrial rate responsive pacing according to thesensor indicated rate determined from the physiological sensor signaland indicative of the patient's metabolic demand, e.g., a patientactivity signal from an accelerometer included in sensors 212.Similarly, sensors 212 included in RV pacemaker 14 may include aphysiological sensor producing a signal indicative of the patient'smetabolic demand, and the control module 206 may establish a ventricularlower rate interval in response to determining a sensor indicated ratebased on the physiological signal.

RA pacemaker 12 and RV pacemaker 14 may use the same or differentphysiological sensors and/or algorithms for producing a signalindicative of the patient's metabolic demand, determining a sensorindicated pacing rate, and establishing a lower rate interval based onthe sensor indicated pacing rate for controlling rate responsive pacingin the respective atrial and ventricular chambers. The use of a patientactivity signal for providing rate-responsive pacing is generallydisclosed in U.S. Pat. No. 7,031,772 (Condie, et al.), incorporatedherein by reference in its entirety.

A physiological signal produced by sensors 212 may additionally oralternatively be used by sensing module 204 and/or control module 206 todetect mechanical activity of the patient's heart, such as motion of aheart chamber or heart sounds. For example, when pacemaker 100 ispositioned in the RA as the RA pacemaker 12, a signal artifact on anaccelerometer signal due to ventricular contraction at the onset of theventricular systolic ejection phase, may be identified as a FFventricular event and used as a surrogate for sensing electrical FFventricular events by FF event detectors 224.

In other examples, an acoustical sensor may be included in sensors 212for producing a signal comprising heart sound signals. Sensing module204 may be configured to sense heart sounds from the acoustical signalas evidence of far field ventricular events. RA pacemaker 12 may includean acoustical sensor for use in sensing heart sounds and providingcontrol module 206 with a FF ventricular event sense signal indicating amechanical ventricular event has occurred. For example, S1 heart soundsmay be sensed as a mechanical surrogate for the electrical sensing ofventricular pacing pulses or R-waves.

In some examples RA pacemaker 12 may be configured to sense bothelectrical FF ventricular events and mechanical FF ventricular events.For example, the S1 heart sound may be used to confirm an electrical FFventricular sense signal when the S1 heart sound is detected within apredetermined time interval following an electrical FF ventricularevent. In another example, providing redundancy of sensing FFventricular events may avoid undersensing of ventricular events. Amechanical FF ventricular event may be sensed based on detection of theS1 heart sound and used to set an atrial pacing escape interval even ifan electrical FF ventricular event was undersensed by FF event detector224.

Whether control module 206 receives a signal from sensors 212 based onFF mechanical ventricular events or from sensing module 204 based on FFelectrical ventricular events, control module 206 of the RA pacemaker 12may use a FF ventricular event signal to set atrial pacing escapeintervals to achieve atrial pacing that is coordinated with ventricularevents. When RA pacemaker control module 206 is configured to set anatrial pacing escape interval timer to coordinate atrial pacing pulseswith ventricular events based on mechanical FF ventricular events, thetime intervals used by the control module 206 to set the atrial escapeinterval timer are adjusted accordingly to account for relativedifferences in timing of electrical and mechanical ventricular events.

Power source 214 provides power to each of the other modules andcomponents of pacemaker 100 as required. Control module 206 may executepower control operations to control when various components or modulesare powered to perform various pacemaker functions. Power source 214 mayinclude one or more energy storage devices, such as one or morerechargeable or non-rechargeable batteries. The connections betweenpower source 214 and other pacemaker modules and components are notshown in FIG. 3 for the sake of clarity.

Telemetry module 208 includes a transceiver and associated antenna fortransferring and receiving data via a radio frequency (RF) communicationlink. RF communication with external device 20 (FIG. 1), may occur inthe Medical Implant Communication Service (MICS) band, the Medical DataService (MEDS) band, or other frequency bands, including, but notlimited to a 2.4 GHz industrial, scientific and medical (ISM) band forBluetooth and IEEE 802.11 b/g/n standards. Telemetry module 208 may becapable of bi-directional communication with external device 20 over awide range of distances, e.g., up to approximately 10 meters. In otherexamples, telemetry communication may require the use of a programminghead placed in proximity of pacemaker 100 to facilitate data transfer.

FIG. 4 is a flow chart 300 of a method for controlling atrial pacingpulse delivery by RA pacemaker 12. Flow chart 300 and other flow chartspresented herein are intended to illustrate the functional operation ofthe device, and should not be construed as reflective of a specific formof software or hardware necessary to practice the methods described. Itis believed that the particular form of software, hardware and/orfirmware will be determined primarily by the particular systemarchitecture employed in the pacemaker 100 and by the particulardetection and therapy delivery methodologies employed by the pacemaker100. Providing software, hardware, and/or firmware to accomplish thedescribed functionality in the context of any modern pacemaker system,given the disclosure herein, is within the abilities of one of skill inthe art. Methods described in conjunction with flow charts presentedherein may be implemented in a computer-readable medium that includesinstructions for causing a programmable processor to carry out themethods described. The instructions may be implemented as one or moresoftware modules, which may be executed by themselves or in combinationwith other software.

At block 302 an atrial lower rate interval (LRI) is established by thecontrol module 206 of RA pacemaker 12. The LRI may initially beestablished as a programmed base rate for providing bradycardia pacing.For example, a LRI may initially be 1,000 ms corresponding to aprogrammed base rate of 60 bpm. A base rate LRI may be set to providebradycardia pacing in the range of 40 to 70 bpm. The LRI established atblock 302 may be adjusted from the programmed base rate according to asensor-indicated rate determined from a physiological signal indicativeof the patient's metabolic demand if rate responsive pacing is enabled.Accordingly, the LRI established at block 302 may be a sensor indicatedrate interval, which is sometimes referred to as a “temporary LRI,” thatis shorter than the base pacing rate interval. The SIR interval isshorter than the base pacing rate interval if an activity sensor signalor other indicator of metabolic demand indicates a higher pacing rate isneeded.

As described in greater detail below, the LRI may additionally beadjusted from a (SIR) interval to equalize the atrial rate and aventricular rate determined from FF ventricular events sensed by RApacemaker 12. The LRI established at block 302 may therefore be aninterval set based on a sensor-indicated rate and a FF ventricular eventrate. In this way, the atrial LRI is adjusted at block 302 in responseto the physiological sensor-indicated rate and the FF ventricular eventrate to match the atrial pacing rate to both the patient's metabolicdemand and the ventricular rate when the ventricular rate is most likelya rate-responsive paced ventricular rate and not a ventriculartachyarrhythmia as further described in conjunction with the flow chartof FIG. 7.

At block 304, an atrial pacing escape interval is set to an AA pacinginterval equal to the LRI established at block 302. An escape intervaltimer set to the atrial pacing escape interval is started at block 304in response to an initial atrial pacing pulse to control the timing ofthe next atrial pacing pulse. In other examples, an initial atrialpacing escape interval may be set equal to the LRI upon sensing thefirst intrinsic atrial event upon RA pacemaker implantation.

If the atrial pacing escape interval expires at block 314 with noventricular events sensed by FF event detector 224 of RA pacemaker 12(or sensors 212) during the atrial pacing escape interval (negativeresult at block 306), and no intrinsic atrial events are sensed by NFevent detector 222 during the atrial pacing escape interval (negativeresult at block 310), an atrial pacing pulse is delivered at block 316at the established LRI. Upon delivering an atrial pacing pulse, thecontrol module 206 of RA pacemaker 12 returns to block 302 to establishthe atrial LRI by making any necessary adjustments based on asensor-indicated rate and/or rate of FF ventricular events. The nextatrial pacing escape interval is set to an AA interval equal to the LRIestablished at block 302.

If a ventricular event is sensed during the atrial pacing escapeinterval, however, as determined at block 306, the control module 206 ofRA pacemaker 12 restarts the atrial pacing escape interval to a VAinterval at block 308. The VA interval is set equal to the LRIestablished at block 302 less a target atrioventricular (AV) interval.The target AV interval may be retrieved from memory 210 of RA pacemaker12. The target AV interval is the interval between an atrial pacingpulse and a subsequent ventricular event that is desired to achieveoptimal timing between the atrial and ventricular contractions. A targetAV interval may be determined for a given patient based on hemodynamicmeasurements or other optimization techniques or may be programmed to anominal value. A programmed target AV interval stored in memory 210 ofRA pacemaker 12 may be in the range of 100 to 300 ms, and more typicallyin the range of 150 to 250 ms. The target AV interval may beautomatically adjusted with changes in the LRI established at block 302to provide a shorter AV interval during episodes of higher pacing rates(shorter LRI) and a longer AV interval during relatively slower pacingrates (longer LRI).

A target AV interval may be stored in memory 210 for subtracting fromthe established LRI for setting a VA interval in response to anelectrical FF ventricular event, and a different target AV interval maybe stored in memory 210 for subtracting from the established LRI forsetting a VA interval in response to a mechanical FF ventricular event.The S1 heart sound may be 10 to 50 ms later than an electrical R-wavefor example. Accordingly, the target AV interval used in setting a VAinterval in response to a mechanical FF ventricular event may be 10 to50 ms shorter than a target AV interval used in setting a VA interval inresponse to an electrical FF ventricular event.

In some cases, the AV interval may be adjusted from a stored orprogrammed AV interval to coordinate the atrial pacing pulses and theventricular pacing pulses. The RA pacemaker 12 may measure an actual AVdelay time between an atrial pacing pulse and a subsequent FFventricular event and adjust the AV interval used in setting the escapeinterval at block 308 to cause the actual AV delay time to match thetargeted AV interval when AV conduction is intact.

If the atrial pacing escape interval (set to the VA interval at block308) expires at block 314 without sensing an intrinsic atrial event bythe NF event detector 222 at block 310, an atrial pacing pulse isdelivered at block 316 by the RA pacemaker 12 at the expiration of theVA interval at block 314. By setting the atrial pacing escape intervalto the LRI less the targeted AV interval, the next ventricular event,intrinsic or paced, will be preceded by an atrial pacing pulse deliveredat approximately the target AV interval prior to the next ventricularevent, assuming the ventricular rate is not changing significantly sincea preceding cardiac cycle and no intervening atrial sense event occurs.In this way, the RA pacemaker 12 is configured to deliverventricular-synchronized atrial pacing pulses that are coordinated withsensed FF ventricular events, electrical or mechanical.

In one example, a scheduled atrial pacing pulse is inhibited if anatrial event is sensed at block 310 during the atrial pacing escapeinterval, which may be either an AA interval set equal to the LRI atblock 304 or a VA interval set equal to the LRI less the target AVinterval at block 308. At block 312, the next atrial pacing pulse isscheduled to occur at the established LRI after the atrial sense eventplus any unexpired time of the currently running escape interval.

The atrial pacing pulse scheduled to occur at the expiration of apreviously started AA or VA interval is inhibited, and the next atrialpacing pulse may be scheduled by re-setting the atrial escape intervaltimer at block 312 upon sensing an atrial event by NF event detector 222in some examples. The atrial pacing escape interval restarted at block312 may be set to the currently established LRI plus all unexpired timeremaining in the previously started atrial pacing escape interval at thetime that the intrinsic atrial event was sensed during the escapeinterval. The unexpired time may be determined by the control module 206and added to the LRI for resetting the escape interval timer, or the LRImay be added to any remaining time left on the escape interval timer atthe time that the intrinsic atrial event is sensed.

As such, if the escape interval timer was set to an AA interval inresponse to an atrial pacing pulse, the escape interval timer is resetin response to a NF atrial sense event signal received by the controlmodule 206 to an atrial pacing escape interval that includes a sum ofthe established LRI and any time remaining in the unexpired AA intervalupon receiving the NF atrial sense event signal from NF event detector222. This situation is illustrated in FIGS. 5A and 5B. If the escapeinterval timer was set to a VA interval in response to a sensed FFventricular event, the escape interval timer is reset in response toreceiving a NF atrial sense event signal by the control module 206 to anatrial pacing escape interval that includes a sum of the established LRIand any time remaining in the unexpired VA interval upon receiving theNF atrial sense event signal from the NF event detector 222. Any timethe atrial pacing escape interval expires at block 314, an atrial pacingpulse is delivered at block 316 and the process returns to block 302 toadjust the LRI if needed and set the next atrial pacing escape intervalto an AA interval equal to the LRI at block 304.

If multiple NF atrial sense event signals are received without anintervening atrial pacing pulse or FF ventricular event, unexpired timeon the atrial escape interval timer may accumulate over multipleintrinsic atrial cycles when the LRI is repeatedly added to anyunexpired time remaining on the escape interval timer in response toeach NF atrial sense event. This situation is described in conjunctionwith FIG. 5B. Accumulation of unexpired time due to consecutively sensedatrial events may result in an inappropriately long atrial escapeinterval. Accordingly, at block 312, the control module 206 may set theatrial pacing escape interval to the established LRI plus unexpired timeof the currently running escape interval within predefined limits orboundaries to avoid accumulation of unexpired escape intervals leadingto an excessively long atrial pacing escape interval.

In one example, a maximum accumulated time limit that can be added tothe established LRI in response to a NF atrial sense event signal may bedefined. For example, a maximum accumulated unexpired time added to theestablished LRI may be equal to the established LRI so that the escapeinterval set at block 312 is never greater than twice the establishedLRI.

In another example, the atrial pacing escape interval set at block 312may be set to an expected time of the next FF ventricular event of thenext cardiac cycle less the target AV interval. If AV conduction is notintact, the ventricular rate may continue at a paced rate according toan established ventricular LRI. The NF atrial sensed event will not beconducted to the ventricles, but the next ventricular pacing pulse canbe expected to occur at the established ventricular LRI. The expectedtime of the FF ventricular event of the next cardiac cycle may be basedon previously measured intervals between FF ventricular events or theestablished atrial LRI assuming it approximately matches the ventricularLRI. Generally, the atrial pacing escape interval is set in response toa NF atrial sense event to provide an atrial pacing pulse at a VAinterval after the next expected FF ventricular event of the currentcardiac cycle or a target AV interval before the FF ventricular event ofthe next cardiac cycle.

In another example, at block 312 the control module 206 schedules thenext atrial pacing pulse to occur at the atrial LRI plus any unexpiredtime of the current atrial pacing escape interval at the time the atrialevent was sensed by allowing the unexpired atrial pacing escape intervalto expire. At the expiration of the atrial pacing escape interval, thescheduled atrial pacing pulse is withheld, and the next atrial pacingescape interval is set equal to the atrial lower rate interval. Thisprocess of allowing the atrial pacing escape interval to expire,withholding a scheduled atrial pacing pulse and starting a next atrialpacing escape interval equal to the established LRI upon expiration ofthe atrial pacing escape interval allows atrial pacing pulses to bescheduled at regular intervals. Examples of atrial pacing escapeintervals set in response to a NF atrial sensed event using thistechnique are described in FIG. 5C.

The VA interval started in response to a FF ventricular eventcontributes to the regulation of an actual AV delay within range of atarget AV interval. The VA interval set as FF ventricular events aresensed also acts to prevent unexpired time from atrial pacing escapeintervals from accumulating when consecutive NF atrial events are sensedwithout atrial pacing. Examples of setting a VA interval are describedin conjunction with FIG. 6.

FIG. 5A is a timing diagram 500 illustrating one method for controllingatrial pacing pulse delivery by RA pacemaker 12. An initial atrialpacing pulse 502 causes the atrial pacing escape interval timer to beset to the atrial LRI 504 established by the RA pacemaker 12. The atrialLRI 504 may be set to the base pacing rate interval or set according toa sensor-indicated rate. When set according to a sensor-indicated rate,the atrial LRI 504 may be further adjusted to equalize the atrial pacingrate with a rate of FF ventricular events as described in conjunctionwith the flow chart of FIG. 7.

The next atrial pacing pulse 506 is delivered upon expiration of atrialLRI 504, causing the atrial pacing escape interval timer to be reset tothe atrial LRI 508. An atrial sensed event 510 occurs during the atrialLRI 508, causing the atrial escape interval timer to be reset withoutdelivering an atrial pacing pulse. The new atrial escape interval 514set in response to the atrial sensed event 510 includes a portion 514A,equal to the unexpired time 512 of the previously started atrial LRI508, plus a portion 514B equal to the established atrial LRI 504. Thenext atrial pacing pulse 520 is delivered upon expiration of thisextended escape interval 514.

The extended escape interval 514 is set equal to the unexpired portion512 of the previous atrial escape interval 508 plus the atrial LRIportion 514B in order to maintain a target AV interval 540 and topromote a regular ventricular rate (when AV conduction is intact).Atrial sense event 510 may be a premature atrial contraction, retrogradeconduction of a ventricular event or other event which is not used bythe RA pacemaker 12 to drive the atrial rate. Rather, it is used toschedule an atrial pacing pulse 520 at the target AV interval 540 aheadof an expected ventricular event 538 on the next cardiac cycle.

In the example shown, there is no intrinsic AV conduction such that RVpacemaker 14 is pacing the ventricle independently of atrial activity.RV pacemaker 14 may be delivering ventricular pacing pulses 530, 534,536, and 538 at a stable ventricular LRI 532. The ventricular LRI 532 isset equal to a ventricular LRI established by the RV pacemaker 14, whichmay be a programmed base rate interval or shortened from the base rateinterval to a SIR interval. The ventricular pacing pulses 530 and 534arrive at a target AV interval 540 following the atrial pacing pulses502 and 506.

If the RV pacemaker pacing rate is constant at the ventricular LRI 532,ventricular pacing pulse 536 arrives after the atrial sensed event 510at an actual AV delay time interval 542 that is much longer than thetarget AV interval 540. By setting the extended escape interval 514following the atrial sensed event 510, however, the next atrial pacingpulse 520 is delivered at the target AV interval 540 prior to theventricular pacing pulse 538 in the next cardiac cycle.

In this way, even if sensing of the FF ventricular events (i.e., pacingpulses 530, 534, 536 and 538, evoked R-wave associated therewith, orassociated mechanical events) intermittently disappears, the RApacemaker 12 is able to maintain atrial pacing coordinated with theregular ventricular rate determined before sensing of FF ventricularevents was lost by setting the extended escape interval 514 in responseto atrial sense event 510. Atrial pacing pulse 520 is delivered atapproximately the target AV interval 540 prior to the expected FFventricular event corresponding to ventricular pacing pulse 540 on thenext cardiac cycle after the NF atrial sense event 510.

If AV conduction returns, ventricular depolarizations conducted from theatria may cause ventricular pacing to be inhibited. If the early atrialsense event 510 is conducted to the ventricles, a ventricular event 548may occur after an intrinsic AV conduction time that does not match thetarget AV interval. If this occurs, the ventricular rate increases forone cardiac cycle, and the ventricular pacing pulse 536 is inhibited.However, a regular ventricular rate will be resumed on the next cardiaccycle by applying the extended atrial escape interval 514 that includesan unexpired portion 512 of the previous atrial escape interval 508.

If the intrinsic atrial rate is actually increasing, e.g., during sinustachycardia, consecutive atrial sensed events will inhibit the atrialpacing pulses. If AV conduction is intact, the ventricular ratenaturally follows the atrial rate. If AV conduction is blocked, the RVpacemaker adjusts the ventricular LRI according to a sensor-indicatedrate. The RA pacemaker 12 will track the rate of the FF ventricularevents and if the atrial sinus rate is lagging or leading the FFventricular event rate, the atrial LRI is adjusted to match the FFventricular event intervals according to techniques described below inconjunction with FIG. 7.

FIG. 5B is a timing diagram 550 illustrating a method for setting atrialpacing escape intervals in response to atrial sense events in theabsence of FF ventricular event sensing according to another example.Atrial pacing pulses 552, 556 are delivered at an established atrial LRI554. Ventricular pacing pulses 594 are delivered at an establishedventricular LRI 592. The atrial pacing pulses 552 and 556 arrive at thetarget AV interval 590 prior to the ventricular pacing pulses 594.Atrial sense event 558 occurs during the atrial LRI 560, which may causethe atrial pacing escape interval timer to be reset to an atrial pacingescape interval 566 that includes a first portion 564 equal to theunexpired portion 562 of atrial LRI 560 plus a second portion 565 equalto the atrial LRI 554.

Another atrial sense event 571 is sensed during the second portion 565of atrial pacing escape interval 566. An unexpired portion 572 of escapeinterval 566 remains at the time of atrial sense event 571. A new atrialpacing escape interval 576 is started including a portion 574 equal tothe unexpired time 572 of the previous escape interval 566 added to aportion 582 equal to the atrial LRI 554. The total atrial pacing escapeinterval 576 includes unexpired time 572 that is an accumulation ofunexpired time of the atrial LRI during the preceding two escapeintervals 560 and 566 during which atrial sense events 558 and 571occur.

Upon expiration of atrial escape interval 576, an atrial pacing pulse588 is delivered. Atrial pacing pulse 588 occurs at the target AVinterval 590 ahead of the ventricular pacing pulse 596 in this examplebecause the unexpired portions 562 and 572 of the previous two pacingescape intervals 560 and 566 have been tracked by incorporating theseunexpired LRI portions 562 and 572 in the atrial pacing escape interval576. If the ventricular rate has not changed, which in this example hasbeen controlled by a constant ventricular LRI 592, the atrial pacingpulse 588 arrives at the target AV interval 590 ahead of ventricularpacing pulse 596 achieving atrial-ventricular synchrony despitewithholding atrial pacing pulses for two atrial cycles and withoutsensing FF ventricular events.

As observed in FIG. 5B, the unexpired times 562, 572 of the atrialpacing escape intervals 560, 566 may accumulate in the next escapeinterval 576 when consecutive atrial sense events occur at intervalsshorter than the atrial LRI without any ventricular sense events. Ifthis process of adding unexpired time to the LRI in response tosuccessive atrial sense events continues over an extended period oftime, however, the accumulated unexpired time of previous atrial pacingescape intervals added to the atrial LRI 554 may lead to an excessivelylong atrial pacing escape interval. If no intrinsic atrial events occurduring the excessively long atrial pacing escape interval, a period ofatrial asystole could result. In order to avoid an excessively longatrial pacing escape interval, the extended escape intervals 566, 576may be limited to a maximum of twice the atrial LRI 554 in one example.

In other examples, therefore, control module 206 may extrapolate theexpected time of a future ventricular event based on previouslydetermined sensed FF ventricular event intervals and schedule an atrialpacing pulse according to an expected ventricular event time. During anextended period of atrial sensing in the absence of intrinsic AVconduction, however, the ventricular rate could change such that when anatrial pacing pulse is ultimately delivered, it may not arrive within anacceptable range of the target AV interval 590 of a ventricular pacingpulse.

FIG. 5C is a timing diagram 550′ illustrating an alternative method forsetting atrial pacing escape intervals in the presence of atrial senseevents and absence of FF ventricular event sensing. Timing diagram 550′includes the identically-numbered atrial pacing pulses 552 and 556delivered at atrial LRI 554, ventricular pacing pulses 594 delivered ata ventricular LRI 592, and atrial sense events 558 and 571 as shown inFIG. 5B. A third atrial sense event 581 is shown to occur consecutivelyafter atrial sense events 558 and 571. In this example, the controlmodule 206 does not restart the atrial pacing escape interval timer inresponse to atrial sense events 558, 571, and 581.

The atrial pacing escape interval 560 started upon delivery of atrialpacing pulse 556 is allowed to expire even though an atrial sense event558 occurs during the escape interval 560. The control module 206,however, inhibits the atrial pacing pulse scheduled at the expiration570 of atrial pacing escape interval 560 in response to atrial senseevent 558. The control module 206 restarts the pacing escape intervaltimer at the expiration 570 of escape interval 560. The pacing escapeinterval timer is set to a new atrial pacing escape interval 568 equalto the atrial LRI 554.

In response to atrial sense event 571, which occurs during atrial pacingescape interval 568, the pacing pulse scheduled at the expiration 573 ofescape interval 568 is withheld. A new pacing escape interval 569 equalto the atrial LRI 554 is started at expiration 573. A second atrialsense event 581 occurs during atrial pacing escape interval 568. Thesecond atrial sense event 581 is ignored for purposes of controlling thepacing escape interval timer. If more than one atrial sense event occursduring a single atrial pacing escape interval 568, the additional senseevent signals may be ignored; no additional response to the atrial senseevents beyond the first atrial sense event is taken. The pacing escapeinterval 568 continues running, and the scheduled pacing pulse at itsexpiration 573 remains inhibited.

At the expiration of pacing escape interval 568, no pacing pulse isdelivered, but a new pacing escape interval 569 equal to the atrial LRI554 is started. This escape interval 569 expires resulting in deliveryof atrial pacing pulse 588 by pulse generator 202. In this way, atrialsense events 558, 571 and 581 do not disrupt the regularity of theatrial pacing escape intervals 560, 568 and 569 each set to the atrialLRI but do inhibit atrial pacing pulses to avoid a sequence of combinedintrinsic and paced intervals that lead to an overall fast atrial rate.When an atrial escape interval 569 eventually expires, the atrial pacingpulse 568 is delivered at the target AV interval 590 ahead ofventricular pacing pulse 596 if the ventricular rate has not changed,which in this example is controlled by a constant ventricular LRI 592.

FIG. 6 is a timing diagram 600 illustrating a method for controllingatrial pacing pulse delivery by RA pacemaker 12 according to anotherexample. In the example of FIG. 6, the RA pacemaker 12 is sensing FFventricular events 606 and 614. Atrial pacing pulse 602 causes thecontrol module 206 of RA pacemaker 12 to set the atrial escape intervaltimer to start an atrial LRI 604 equal to the established LRI. Duringthe atrial LRI 604, however, a FF ventricular event 606 is sensed by RApacemaker 12, causing the atrial escape interval timer to be reset fromthe started atrial LRI 604 to a VA escape interval 608. VA escapeinterval is set equal to the established atrial LRI 604 less the targetAV interval 640. The target AV interval 640 subtracted from the LRI toset the VA escape interval 608 may take into account delays between anactual ventricular event and the time that the FF ventricular event 606is sensed by the RA pacemaker 12.

Upon expiration of the VA escape interval 608, atrial pacing pulse 610is delivered. An atrial LRI 612 is started in response to the atrialpacing pulse 610. Another sensed FF ventricular event 614 causes theescape interval timer to be reset to a VA escape interval 616 equal tothe established atrial LRI less the target AV interval 640. Both atrialpacing pulses 602 and 610 are delivered at a target AV interval 640prior to the subsequent ventricular pacing pulses 630 and 634,respectively.

Prior to expiration of VA escape interval 616, an intrinsic atrial event618 is sensed by RA pacemaker 12. The atrial sensed event 618 causes theatrial escape interval timer to be reset to an extended escape interval624 that includes first portion 624A equal to the unexpired portion 620of the VA escape interval 616 at the time the atrial event 618 wassensed and a second portion 624B equal to the established atrial LRI,which may be based on an atrial sensor-indicated rate and adjusted fromthe atrial sensor-indicated rate to match the ventricular rate asdescribed below. By extending the escape interval 624 by the unexpiredportion 620 of the previous VA escape interval 616, the next atrialpacing pulse 628 occurs at the target AV interval 640 prior to the nextventricular pacing pulse 642, which is delivered at the ventricular LRI632.

In this example, the ventricular pacing pulse 636 is not sensed as a FFventricular event by RA pacemaker 12. It occurs at an actual AV delaytime interval 642 following the atrial sensed event 618 that is longerthan the target AV interval 640. By setting an extended escape interval624, however, the target AV interval 640 is re-established on the nextcardiac cycle. If the ventricular pacing pulse 636 is sensed by the RApacemaker 12 as a FF ventricular event, the escape interval 624 isrestarted as a VA interval equal to the atrial LRI less the target AVinterval to also arrive at an atrial pacing pulse 628 delivered at thetarget AV interval 640 prior to ventricular pacing pulse 642.

In an alternative example, if an atrial sense event 618 occurs duringthe VA interval 616, the VA interval 616 is allowed to expire, and a newatrial pacing escape interval 625 set equal to the established atrialLRI is started upon the expiration of VA interval 616. The atrial pacingpulse that was scheduled to occur at the expiration of the VA escapeinterval 616 is withheld. The next atrial pacing pulse 628 is scheduledto occur at the expiration of the atrial LRI (escape interval 625) plusany unexpired time 620 of the currently running VA escape interval 616at the time of the atrial sensed event 618 by allowing VA escapeinterval 616 to expire, withholding the scheduled pacing pulse, andstarting an AA escape interval 625 set to the established LRI.

The methods depicted by the timing diagrams 500, 550, 550′ and 600 ofFIGS. 5A, 5B, 5C and FIG. 6 promote regular atrial and ventricular ratessynchronized at a target AV interval even when an intrinsic atrial eventis occasionally sensed during an atrial escape interval set to an AA orVA interval.

Some techniques disclosed in conjunction with the flow charts and timingdiagrams presented herein for controlling an atrial pacing escapeinterval timer of an atrial intracardiac pacemaker may be used forcontrolling a ventricular pacing escape interval timer of a ventricularintracardiac pacemaker. For instance, a ventricular pacemaker sensingmodule may be configured to sense near field events occurring in theventricle and far-field events occurring in the atria. The ventricularpacemaker may be configured to respond to sensed events by withholding ascheduled pacing pulse in the ventricle and controlling a ventricularpacing escape interval timer based on the timing of near-field andfar-field events sensed in the ventricle.

FIG. 7 is a flow chart 400 of a method for controlling RA pacemaker 12to provide atrial pacing coordinated with ventricular events. RApacemaker 12 may be configured to deliver atrial pacing pulses, sense NFintrinsic atrial events and FF ventricular events (dual chamber sensing)and inhibit atrial pacing pulses in response to sensing NF intrinsicatrial events and schedule atrial pacing pulses in response to sensingFF ventricular events. By providing dual chamber sensing in the RApacemaker 12, the RA pacemaker 12 can track ventricular events toprovide coordinated dual chamber pacing in an IMD system that includesseparate RA pacemaker 12 and RV pacemaker 14 without requiring wirelesstelemetry communication between the telemetry modules 208 of therespective RA and RV pacemakers 12 and 14 and without requiring FFsensing of atrial events by the RV pacemaker 14. The timing of theatrial pacing pulses, as described above, may be controlled by settingan escape interval timer to a VA interval when an electrical FFventricular event is sensed by FF event detector 224, which may be aventricular pacing pulse or an R-wave, or when a mechanical FFventricular event is sensed by sensors 212. The FF sensed event may be aphysiological event, either intrinsic or evoked, or a therapeutic pacingpulse delivered to pace the ventricle that is not a communication signalor other non-therapeutic signal transmitted from the RV pacemaker 14 tothe RA pacemaker 12.

The RV pacemaker 14 may be programmed for delivering ventricular pacingpulses, sensing ventricular intrinsic events, and inhibiting ventricularpacing pulses in response to sensing a ventricular intrinsic event. WhenAV conduction is intact, RA pacemaker 12 will drive the atrial rate andthe ventricular rate will follow through the normal conduction system ofthe heart. When AV conduction is lost, RV pacemaker 14 will drive theventricular rate, which may be a programmed lower base rate when rateresponsive pacing is disabled or a sensor-indicated rate when rateresponsive pacing is enabled. The RV pacemaker 14 will operateindependent of the RA pacemaker 12 when intrinsic AV conduction isblocked. The RA pacemaker 12, however, will track the rate of FFventricular events when the rate of the FF ventricular events is below amaximum tracking rate.

In order to maintain coordinated atrial and ventricular rates if AVconduction is lost, the control module 206 of RA pacemaker 12 isconfigured to determine and compare a rate of FF ventricular events anda rate of atrial events (paced and/or sensed) by determining AA and FFventricular intervals (i.e., W intervals) at block 402. If the atrialrate is less than (or greater than) the ventricular rate, AV conductionmay be lost and the RV pacemaker 14 may be pacing the ventricle at ahigher (or lower) rate than the intrinsic or paced atrial rate.

AA and W intervals may be determined and compared beat-by-beat, bydetermining and comparing a median or mean of N consecutive intervals,by determining a running average of N consecutive intervals or othermethods. In some examples, the rates may be considered equal if adetermined AA interval measurement and a determined VV intervalmeasurement are within a threshold difference of each other, e.g.,within 10 ms of each other. However, a small, sustained differencebetween AA and W intervals can accumulate over time causing a shift ofthe atrial events relative to the ventricular events such that a targetAV interval is lost. One or more atrial pacing pulses at a modified AAinterval(s) may be delivered to bring the actual AV delay back within anacceptable range of a target AV interval, e.g., according to the flowchart of FIG. 7.

If the atrial rate is less than the ventricular rate, as determined atblock 404, the control module 206 of RA pacemaker 12 determines if theatrial rate response function is active at decision block 406 asevidenced by an atrial sensor-indicated rate that is greater than theprogrammed base rate. If the atrial rate response function is active,indicating that a pacing rate greater than the programmed base pacingrate is warranted, the RV pacemaker rate response function is likely tobe active as well. The RV pacemaker rate response function, however, maybe producing a sensor-indicated rate that is greater than thesensor-indicated rate determined by the RA pacemaker 12. It is desirableto control the RA pacemaker 12 to track the higher ventricular rate tomaintain coordinated atrial and ventricular activity as long as thehigher ventricular rate is duly warranted based on patient metabolicdemand and not due to ventricular tachyarrhythmia.

Accordingly, if the atrial rate response is inactive as indicated by aSIR that is not greater than the programmed base pacing rate (at block406), the faster FF ventricular event rate may be due to ventriculartachyarrhythmia. No adjustment to the atrial LRI is made at block 410.The RA pacemaker 12 will not track the faster ventricular rate when theRA pacemaker control module 206 determines that an increased pacing rateis unwarranted based on the atrial rate response function beinginactive, i.e., not producing a sensor-indicated rate greater than thebase pacing rate.

If the atrial rate response function is actively producing asensor-indicated rate greater than the programmed atrial base pacingrate as determined at block 406, the RA pacemaker control module 206determines if the rate of FF ventricular events sensed by the RApacemaker 12 is less than a ventricular tracking upper rate limit (block408). A ventricular tracking upper rate limit may be programmed in RApacemaker memory 210. In one example, if the ventricular rate is greaterthan the tracking upper rate limit, there is no adjustment to the atrialLRI being used by the RA pacemaker 12 (block 410). The FF ventricularevent rate that is determined to be greater than the atrial rate atblock 404 may be due to ventricular tachyarrhythmia, and thereforeatrial tracking of the higher ventricular rate may be undesirable.

The RA pacemaker 12 may be configured to control the atrial pacing rateto track a rate of FF ventricular events up to upper rate limit byshortening the atrial LRI at block 412. The upper rate limit may be ashigh as 120 to 140 bpm, for example. In practice, the upper rate limitmay less than 120 bpm, e.g., 100 bpm, since FF ventricular event sensingmay be less reliable at higher heart rates and because AV synchrony istypically more important at rest. If the rate of FF ventricular eventsexceeds the upper rate limit, the RA pacemaker 12 continues to use theatrial LRI established according to the sensor indicated rate determinedby the control module 206 of RA pacemaker 12 without adjustment of theatrial LRI in response to the higher FF ventricular event rate (a directpath from block 408 to block 410 not shown in FIG. 7).

Alternatively, if the FF ventricular events exceed an upper rate limit,at block 408, the atrial LRI may be decreased at block 413 to equalizethe atrial and ventricular rates, but a minimum AA interval limit may beset. Atrial pacing pulses may be delivered at the expiration of a VAinterval set based on the decreased atrial LRI when a FF ventricularevent is sensed as long as the resulting actual AA interval is greaterthan a minimum AA interval limit. In this way, a maximum atrial rate iscontrolled by inhibiting atrial pacing pulses during a fast ventricularrate only when an atrial pacing pulse would result in an actual AAinterval less than a minimum limit. To illustrate, atrial pacing pulsesmay be delivered at the expiration of every other VA interval whenatrial pacing synchronized to every FF ventricular event results in anactual AA interval that is less than a minimum AA interval limit. Inthis way, atrial pacing pulses that are delivered are synchronized toventricular events during a faster ventricular rate, but anunacceptable, pacing-induced fast atrial rate is avoided.

If the FF ventricular event rate is less than the upper rate limit atblock 408, the FF ventricular event rate may be a valid sensor-indicatedrate produced by the control module 206 of the RV pacemaker 14. Theventricular sensor-indicated rate produced by the RV pacemaker 14 may behigher than the sensor-indicated rate produced by the RA pacemaker 12,causing the atrial rate to be less than the ventricular rate. If thesensor-indicated rate produced by the RV pacemaker 14 is slightlygreater than the sensor-indicated rate produced by the RA pacemaker 12,this discrepancy may cause the atrial pacing rate to lag the ventricularpaced rate, resulting in uncoordinated atrial and ventricular activity.The increased FF ventricular event rate, however, is determined to bewarranted due to an active rate response of the RA pacemaker 12producing a SIR greater than the base rate and not likely due toventricular tachyarrhythmia.

In this situation, atrial tracking of the ventricular rate is desirable.At block 412, the control module 206 of RA pacemaker 12 decreases(shortens) the currently established atrial LRI. The atrial LRI may bedecreased in a step-wise manner according to the step resolution of theatrial pacing escape interval timer of the RA pacemaker 12.Alternatively, a difference interval that is the difference between oneor more FF ventricular event intervals (or an averaged FF ventricularevent interval) and one or more AA intervals (or an averaged AAinterval) may be determined at block 412. The atrial LRI may bedecreased by the determined difference interval so that the FFventricular event intervals and the AA intervals are approximatelyequal, e.g., within 10 ms of each other.

The process then returns to block 402 to determine the AA and FFventricular event intervals again. If the atrial rate is still less thanthe ventricular rate after adjusting the atrial LRI, the atrial LRI maycontinue to be adjusted at block 412, as long as the FF ventricularevent rate remains below a ventricular tracking upper rate limit, untilthe FF ventricular event rate is no longer greater than the atrial rate,as determined by a negative result at decision block 404.

FIG. 8 is a timing diagram 700 of pacing pulses delivered by RApacemaker 12 and RV pacemaker 14 illustrating the operation of RApacemaker 12 for controlling atrial pacing pulse delivery when thesensed FF ventricular event rate is faster than the atrial rate (“yes”branch of block 404 in flow chart of FIG. 7). Atrial pacing pulses 702and 712 are delivered at an established atrial LRI 703. Atrial LRI 703may be based on a sensor-indicated rate by the control module 206 of RApacemaker 12.

Ventricular pacing pulses 750 and 754 are delivered at the ventricularLRI 752 established by RV pacemaker 14. The FF ventricular event rateinterval 706 may be stable prior to atrial pacing pulse 702 and up toatrial pacing pulse 712 due to a stable ventricular LRI 752. The atrialpacing pulses 702, 712 and 726 start the atrial pacing escape intervaltimer at the AA intervals 704, 705, and 722 set equal to the establishedatrial LRI 703. The atrial pacing escape interval timer is restartedduring each of these AA intervals 704, 705 and 722 in response tosensing the respective FF ventricular events 708, 718, and 719. Uponeach FF ventricular event 708, 718, and 719, the atrial pacing escapeinterval is set to a VA escape interval 710, 716, and 724, respectively,which is equal to the atrial LRI 703 less the target AV interval 756.Atrial pacing pulses 726 and 728 are delivered at the expiration of theVA escape intervals 716 and 724 set based on the atrial LRI 703. Asdescribed below, a change in the ventricular rate is detected by RApacemaker 12 between atrial pacing pulse 726 and atrial pacing pulse728. The next VA escape interval 734 is set based on an adjusted atrialLRI and is therefore shorter than the preceding VA escape intervals 710,716 and 724.

The ventricular LRI 752 is adjusted to a shorter LRI 758 followingventricular pacing pulse 754, e.g., in response to a sensor indicatedpacing rate determined by the control module 206 of the RV pacemaker 14.Ventricular pacing pulse 760 is delivered upon expiration of aventricular pacing escape interval set equal to the adjusted ventricularLRI 758 and is sensed as a FF ventricular event 720. FF ventricularevent 720 occurs early in the target AV interval 756 due to theshortened ventricular LRI 758. Likewise, the next ventricular pacingpulse 764 occurs early in the target AV interval 756 due to thediscrepancy between the previously established atrial LRI 703 used toset the VA escape interval 724 and the newly shortened ventricular LRI758.

As described in conjunction with the flow chart 400 of FIG. 7, RApacemaker 12 monitors the FF ventricular event rate based on determiningthe FF ventricular event intervals 706, 714, 730. While only twoshortened FF ventricular event intervals 714 and 730 are shown, it isunderstood that the RA pacemaker 12 may monitor more than two FFventricular event intervals before adjusting the atrial LRI to match theFF ventricular event rate. In this example, as few as two FF ventricularevent intervals 714 and 730 are used to detect an atrial rate that isslower than the FF ventricular event rate. The control module 206 of theRA pacemaker 12 determines an AA interval (AAI) 780 (shown in thecenter, inset box), which is equal to the LRI 703 during ongoing atrialpacing in this example. AAI 780 is compared to a W interval (WI) 782that is determined from FF ventricular event intervals 714 and 730. Anatrial rate slower than the ventricular rate is detected in response tothe comparison of the AA interval 780 and the W interval 782. Adifference interval 784 may be determined as the difference between theAA interval 780 and the VV interval 782. The atrial LRI 703 is decreasedby the difference interval 784 to an adjusted atrial LRI that matchesthe determined FF ventricular event interval 782 (and newly establishedventricular LRI 758).

This shorter adjusted atrial LRI is used to set the next atrial escapeinterval 734 in response to the sensed FF ventricular event 732. Thisatrial escape interval 734 is set as a VA interval that is equal to theadjusted atrial LRI less the target AV interval 756. The next atrialpacing pulse 736 is delivered at a shortened AA interval 738 due to therelatively early sensed FF ventricular event 732 starting the VA escapeinterval 734, now set using the shortened atrial LRI. This results inatrial pacing pulse 736 being delivered at the target AV interval 756prior to the next ventricular pacing pulse 768. On the next cardiaccycle, the atrial rate indicated by AA interval 740 (equal to theadjusted atrial LRI) matches the FF ventricular event interval 730 andcorresponding ventricular LRI 758. The atrial pacing pulse 742 deliveredat the expiration of the VA escape interval 744 set based on theadjusted atrial LRI arrives prior to the ventricular pacing pulse 770 atthe target AV interval 756. In this way, the RA pacemaker 12 controlsatrial pacing pulse delivery to match a ventricular rate determined fromsensed FF ventricular events and restores the target AV interval 756prior to ventricular pacing pulses when the ventricular rate acceleratesahead of the atrial rate. As described in conjunction with the flowchart 400 of FIG. 7, the RA pacemaker operation for shortening theestablished atrial LRI to match the atrial rate to a faster FFventricular event rate is performed when the atrial rate responsefunction is active and the FF ventricular event rate is below an upperrate limit.

Returning to the flow chart 400 of FIG. 7, the operation of the RApacemaker 12 during the opposite situation of the atrial rate beingfaster than a sensed FF ventricular event rate (as determined atdecision block 414) is now described. In response to an affirmativeresult at block 414, the control module 206 of RA pacemaker 12determines if the AA intervals used to determine the atrial rate includeatrial paced intervals. If at least one AA interval is a paced intervalas determined at block 416, the process may advance to block 420. Inother examples, if at least a majority of the AA intervals used for therate comparison that resulted in the atrial rate being faster than theFF ventricular event rate are paced intervals, the process advances toblock 420. In still another example, all of the AA intervals used in thecomparison at block 414 that resulted in a determination of the atrialrate being greater than the FF ventricular event rate may be required tobe paced intervals in order to advance to block 420.

If the AA intervals used to determine the atrial rate do include arequired number of atrial paced intervals (affirmative result at block416), an adjustment to the atrial LRI may be used to slow down theatrial rate that is currently running faster than the FF ventricularevent rate. At block 420, the control module 206 of RA pacemaker 12determines if the atrial rate response function is active, i.e.,producing a sensor-indicated rate that is greater than the programmedbase rate. If the atrial rate response function is not active, i.e., ifa sensor-indicated rate is not being produced by the control module 206for driving the atrial pacing rate higher than the programmed base rate,then no adjustment is made to the atrial pacing control parameters atblock 422. The atrial pacing continues at the currently established LRI,which is the base pacing rate when atrial rate response is inactive. Theatrial pacing rate is not slowed below a base pacing rate if the FFventricular event rate is slower than the atrial pacing rate. The FFventricular event rate determined to be slower than the atrial rate byRA pacemaker 12 may be due to undersensing of FF ventricular events bythe RA pacemaker sensing module 204. In some cases, a discrepancy mayexist between the lower base pacing rates programmed in the RV pacemaker14 and in the RA pacemaker 12, or other therapy control parameters mayexist causing the slower FF ventricular event rate.

If the atrial rate is being driven by a sensor-indicated rate that isabove the programmed base pacing rate (“yes” branch of block 420), theatrial LRI is lengthened at block 424 to slow the paced atrial rate tobring it within an acceptable range of or equal to the FF ventricularevent rate. The atrial LRI may be increased gradually in a step-wisemanner at block 424 until the atrial rate is no longer greater than theventricular rate (as determined by returning to block 402).Alternatively, a difference interval between AA intervals and FFventricular event intervals may be determined at block 424. The atrialLRI may be increased (in one or more increments) by the determineddifference interval at block 424 to approximately equalize the atrialand FF ventricular event rates.

If the AA intervals do not include a required number of paced intervalsat block 416, the atrial rate is an intrinsic atrial rate that is fasterthan the rate of sensed FF ventricular events. Atrial pacing cannot beused to slow down the atrial rate; therefore no adjustment to an atrialLRI is made is made at block 418. Atrial pacing is inhibited by thesensed intrinsic atrial activity occurring at a rate faster than thecurrently established atrial LRI. Undersensing of FF ventricular eventsby FF event detector 224 may cause the RA pacemaker control module 206to detect an atrial rate faster than a FF ventricular event rate or thefaster atrial rate may be caused by an atrial tachyarrhythmia.

FIG. 9 is a timing diagram 800 illustrating atrial pacing pulses andventricular pacing pulses delivered by RA pacemaker 12 and RV pacemaker14, and the operation of RA pacemaker 12 when the atrial rate isdetermined to be faster than a FF ventricular event rate (“yes” branchof block 414 in flow chart of FIG. 7). Atrial pacing pulse 802 starts anAA escape interval 804 set equal to a previously established LRI 803. Asdescribed above, the LRI 803 may be established based on asensor-indicated rate determined by the control module 206 of the RApacemaker 12.

The atrial escape interval timer is restarted in response to a sensed FFventricular event 808. The atrial escape interval 804 is reset to a VAescape interval 810 that is equal to the established LRI 803 less atarget AV interval 856. The next atrial pacing pulse 812 is deliveredupon expiration of the VA escape interval 810. The atrial escapeinterval timer is restarted to an AA escape interval 806 equal to theestablished LRI 803, which is again interrupted by a FF ventricularevent 818 that restarts a VA escape interval 816. As long as theventricular pacing rate does not change, atrial pacing pulses 802 and812 are delivered at the target AV interval 856 prior to the ventricularpacing pulses 850 and 854.

If the ventricular pacing rate is decreased, however, e.g., when a newventricular LRI is determined in response to the ventricularsensor-indicated rate, the atrial pacing pulses may fall out of range ofthe target AV interval 856. For example, a newly established ventricularLRI 858 that is longer than the previous ventricular escape interval 852is set in response to ventricular pacing pulse 854. The next ventricularpacing pulse 860 occurs at an actual AV interval 862 that is much longerthan the target AV interval 856 following atrial pacing pulse 826,because of a mismatch between the atrial sensor-indicated rate and theventricular sensor-indicated rate.

The atrial escape interval timer is set to an AA escape interval 822,still equal to the previously established atrial LRI 803, in response tothe atrial pacing pulse 826. A sensed FF ventricular event 820associated with ventricular pacing pulse 860 occurs later in the AAescape interval 822 because the ventricular pacing rate has slowed. A VAescape interval 824 is set in response to the FF ventricular event 820and is still based on the previously established atrial LRI 803, lessthe target AV interval 856. As a result, the atrial pacing pulse 828delivered upon expiration of the VA escape interval 824 occurs at a longAA interval 805 due the relatively late FF ventricular event 820. Atrialpacing pulse 828 occurs prior to the next ventricular pacing pulse 864at an actual AV interval that is much longer than the target AV interval856.

The RA pacemaker 12, however, monitors the rate of the FF ventricularevents 818, 820 and 832 and determines that the atrial LRI 803 needs tobe increased to slow the atrial pacing rate to match the slower FFventricular event rate, assuming the established atrial LRI 803 is notalready at the programmed atrial base pacing rate interval. Asillustrated in the center inset box, the control module 206 of RApacemaker 12 may determine a difference interval 884 between a Winterval 882 determined from one or more FF ventricular event intervals830 and the AA interval 880 equal to the current atrial LRI 803. Theatrial LRI 803 may be increased by the difference interval 884 to slowthe rate of atrial pacing pulses. The increase may be performed in onestep or more gradually in multiple increments of the atrial LRI 803.

The next sensed FF ventricular event 832 causes the atrial pacing escapeinterval timer to be reset to a new VA escape interval 834 that is equalto the adjusted (increased) atrial LRI less the target AV interval 856.As a result, the next atrial pacing pulse 836 delivered upon expirationof VA escape interval 834 occurs at an extended AA interval 838 due tothe increased VA escape interval 834 started relatively late in theatrial cycle upon sensing FF ventricular event 832. The atrial pacingpulse 836 is, however, delivered at the restored target AV interval 856prior to the next ventricular pacing pulse 868.

By the next cardiac cycle, matching atrial and ventricular rates arealso restored. Atrial pacing pulses 836 and 842 occur at the adjustedatrial LRI 840 that matches the FF ventricular event interval 830 equalto the ventricular LRI 858 between ventricular pacing pulses 868 and870. By monitoring for a FF ventricular event rate that is slower thanan atrial rate, the RA pacemaker 12 can quickly res-establishcoordination between atrial pacing pulses and ventricular pacing pulsesby adjusting the atrial LRI when the currently established atrial LRI isshorter than the base pacing rate interval.

Referring again to the flow chart 400 of FIG. 7, the operation of RApacemaker 12 when the atrial and FF ventricular event rates aresubstantially equal will now be described. The negative result at bothof blocks 404 and 414 indicates that the atrial and FF ventricular eventrates match based on comparison between a determined AA interval and adetermined FF ventricular event interval, both of which may be based onone or more respective atrial event intervals and FF ventricular eventintervals. A match between the event rates may be detected when theinterval measurements are within 10 ms of each other, 20 ms of eachother or other predetermined matching range. If the AA and FFventricular event interval comparison(s) indicate a match between theatrial and FF ventricular event rates, the actual AV interval ismonitored at block 430.

In some circumstances, the atrial and ventricular rates may match, butthe actual AV interval may be outside an acceptable range of the targetAV interval. For example, a difference of a few milliseconds that iswithin the “matching range” may accumulate over time to shift the timingof atrial events outside the target AV interval relative to ventricularevents. Normally, a sensed FF ventricular event will cause the atrialpacing escape interval to be set to a VA interval that will schedule thenext atrial pacing pulse at the target AV interval earlier than the nextexpected ventricular event. However, in some instances, a ventricularevent may occur very late in the atrial cycle. An example of thissituation is shown in FIG. 10A.

FIG. 10A is a timing diagram 900 illustrating atrial pacing pulses andsensed FF ventricular events. Atrial pacing pulses 902, 904 and 905 mayeach be delivered upon expiration of a respective VA escape interval911, 913 set in response to a sensed FF ventricular event 910, 912. TheVA escape intervals 911 and 913 are set to the established atrial LRI903 less a target AV interval 901. However, if a FF ventricular event915 is sensed late in the atrial cycle, e.g., at interval 916 followingatrial pacing pulse 905, a VA escape interval 917 started late in theatrial cycle would lead to an atrial pacing pulse 908 delivered at anactual AA interval 907 that is much longer than the established atrialLRI interval 903. The late FF ventricular event 915 may be anover-sensed T-wave, a PVC, or electrical noise for example. In otherinstances, a late-occurring sensed FF ventricular event 915 may be atrue ventricular event that has shifted later in the atrial cycle overmultiple cardiac cycles due to a small difference in atrial andventricular rate in the absence of AV conduction and in the absence ofFF ventricular sensing by RA pacemaker 12 over multiple cycles.

The use of a VA escape interval in response to FF ventricular events asdescribed previously in conjunction with FIG. 6 for controlling atrialpacing pulses may be limited in some examples based on the relativetiming of the FF ventricular event. For instance, a VA escape intervalmay be set in response to sensing a FF ventricular event only if the FFventricular event occurs within a VA time limit of the preceding atrialevent. In the example shown in FIG. 10A, the actual AV interval 916 fromthe most recent atrial event 905 to the FF ventricular event 915 may becompared to a VA pacing limit 919. If the actual AV interval 916 afterthe most recent atrial event 905 is greater than a VA pacing limit 919,the FF ventricular event 915 may be ignored by the RA pacemaker controlmodule 206 for the purposes of resetting the atrial pacing escapeinterval timer to VA escape interval 917.

In some examples, an AA interval set to the atrial LRI 903′ at atrialpacing pulse 905 may be allowed to expire and an atrial pacing pulse(not shown) may be delivered at the expiration of the atrial LRI 903′.In other examples, the atrial LRI 903′ is allowed to expire withoutdelivering an atrial pacing pulse, (inhibited atrial pace, IAP 906. Ifan atrial pacing pulse is delivered at the expiration of atrial LRI903′, it may be conducted to the ventricles during ventricularrepolarization following FF ventricular event 915. Ventriculartachycardia could potentially be induced in some instances. The atrialpacing pulse scheduled at the expiration of the atrial LRI 903′ may,therefore, be withheld in response to the FF ventricular event 915sensed during the LRI 903′ outside a VA limit. The FF ventricular event915 may or may not be used to set a VA interval. In some cases, theatrial LRI 903′ is allowed to expire in response to the FF ventricularevent 915 during the atrial LRI 903′ but outside a VA limit, and a newatrial LRI 903″ is started by the RA pacemaker control module 206 at thetime of the IAP 906 to maintain regular AA intervals until an atrialpacing pulse is delivered or a FF ventricular event is sensed within aVA pacing limit 919.

A VA pacing limit 919 may be a time interval or percentage of thecurrently established atrial LRI or a time interval or percentage of thetarget AV interval. For example if the FF ventricular event 915 is at anactual AV interval 916 that is at least twice as long as the target AVinterval 901, the sensed FF ventricular event 915 does not reset theatrial pacing escape interval to a VA interval. In another example, ifthe FF ventricular event 915 is sensed later than 30% of the atrial LRI903′, the atrial pacing escape interval is not reset to a VA interval inresponse to the FF ventricular event 915. The VA limit 919 may beapplied to an actual AV interval 916 determined between a precedingatrial pacing pulse 905 and a sensed FF ventricular event 915 or betweena preceding atrial sensed event and a sensed FF ventricular event foruse in determining whether the VA interval 917 will be started inresponse to the sensed FF ventricular event 915 during an atrial pacingescape interval 903′ started at the preceding atrial event 905, which isa paced event in this example but could be a sensed event.

FIG. 10B is a timing diagram 900′ illustrating atrial pacing pulses andsensed FF ventricular events as shown in FIG. 10A with the added FFventricular event 918 sensed after atrial pacing pulse 905 at thetargeted AV interval 901. FF ventricular event 918 may be a true FFventricular event sensed prior to the late FF ventricular event 915,which may be an oversensed T-wave or premature ventricular contraction.

If a VA escape interval 914 is started in response to FF ventricularevent 918 that arrived within the VA pacing limit 919, but a second FFVevent 915 is sensed during the VA escape interval 914, the atrial pacingpulse scheduled at the expiration of the VA escape interval 914 may beinhibited (IAP 906′) to avoid a conducted atrial evoked response fromarriving during ventricular repolarization following FF ventricularevent 915 when it is a PVC or true R-wave. A pacing pulse is notdelivered but the scheduled time of the IAP 906′ at the expiration ofthe VA escape interval 914 may be used to re-start the atrial pacingescape interval timer at an atrial LRI 903″.

As indicated above, in some circumstances, a late-occurring FFventricular event 915 may be a valid FF ventricular event, such as anevent that has gradually shifted later in the atrial cardiac cycle. Thissituation could arise if FF ventricular event sensing is lost for aperiod of time, and the ventricular rate is slightly slower than theatrial rate. For example, if the atrial LRI is set to 900 ms andventricular LRI is set to 910 ms, the rates would be determined to matchif the matching range is ±10 ms or more. If FF ventricular event sensingis lost for an interval in time, the ventricular events will occur 10 mslater in each successive atrial cycle. When FF ventricular events aresensed within the VA limit, the VA escape interval will prevent agradually increasing AV interval. However, if FF ventricular eventsensing is lost for a few minutes, when sensing returns the FFventricular event may be sensed later than the VA pacing limit 919. Aone cycle adjustment based on setting a VA escape interval in responseto the late-occurring sensed FF ventricular event 915 may be undesirablesince it could result in a large step change in atrial rate. In thissituation, therefore, more gradual changes to the atrial LRI 903 may bemade to bring the atrial pacing pulses back into range of the target AVinterval 901.

Referring again to flow chart 400 of FIG. 7, if the atrial rate and FFventricular event rate are approximately equal (“no” branch of block414), the RA pacemaker 12 determines the actual AV interval between anatrial event (paced or sensed) and a sensed FF ventricular event atblock 430. The actual AV interval is compared to a VA pacing limit atblock 432 that may be stored in RA pacemaker memory 210. If the actualAV interval is not greater than a VA pacing limit, as determined atblock 432, no atrial LRI adjustment is made. VA pacing escape intervalsare set by the control module 206 of RA pacemaker 12 in response to FFventricular events to regulate the actual AV interval within range ofthe target AV interval. The process returns to block 402 to continuemonitoring the atrial and FF ventricular event rates.

If the atrial rate and the FF ventricular event rate are approximatelyequal and the actual AV interval exceeds the VA pacing limit (“yes”branch of block 432), the RA pacemaker control module 206 adjusts theatrial LRI at block 435 to check for corresponding changes in the FFventricular event rate as evidence of intrinsic AV conduction. If thelate FF ventricular events are due to slow intrinsic AV conduction,adjustment of the atrial LRI will not correct the actual AV interval.Intrinsically conducted R-waves that are sensed as FF ventricular eventswill follow the atrial LRI changes at the same, long intrinsic AVconduction time. If the actual AV interval stays the same, AV block isnot present as determined at block 436. No adjustments to the atrial LRIis made at block 438

If changes in the actual AV interval occur as the atrial rate is alteredat block 435, the sensed FF ventricular events may be associated withventricular pacing pulses being delivered by the RV pacemaker andtherefore evidence of AV block. If AV block is detected at block 436based on a change in the actual AV interval when the atrial rate isaltered, the AA pacing escape interval is altered from the establishedatrial LRI at block 440 until the actual AV interval is within anacceptable range of the target AV interval. The established atrial LRIis not changed and atrial pacing resumes at the atrial LRI afterrestoring the target AV interval. The process returns to block 402 tocontinue monitoring the atrial and FF ventricular event rates formaintaining coordination between atrial pacing delivered by the RApacemaker 12 and the RV pacemaker 14 operating independently of the RApacemaker.

FIG. 11 is a timing diagram 920 illustrating the operation of the RApacemaker 12 when the atrial rate and the FF ventricular rate have beendetermined to match (“no” branch of block 414 in FIG. 7). In thisexample, the atrial LRI 922 is 1000 ms corresponding to a base pacingrate of 60 beats per minute. The target AV interval is 150 ms. Theventricular LRI of the RV pacemaker 14 is also set to 1000 ms such thatFF ventricular events (FFV) 924 are sensed by the RA pacemaker 12 at1000 ms intervals 926. The FF ventricular events 924 are initiallyoccurring late in the atrial cycle, at an actual AV interval 930 of 400ms. The FF ventricular events 924 may arrive late in the atrial cardiaccycle due to gradual progression of a slightly slower ventricular pacingrate during a loss of FF ventricular sensing by the RA pacemaker 12. TheFF ventricular events 924 may also be pacing pulses (or evoked R-waves)arriving late due to a VV escape interval being set in response to apreviously oversensed T-wave or sensed premature ventricular contractionby the RV pacemaker 14.

The first FF ventricular events 924 occur at an actual AV interval 930of 400 ms, greater than the target AV interval of 150 ms and greaterthan a VA pacing limit 919 of, e.g., twice the target AV interval.Accordingly, no VA escape interval is set in response to the FFventricular events 924, and atrial pacing pulses (AP) continue at the1000 ms LRI 922.

The RA pacemaker 12 determines that the AA intervals 922 and FFVintervals 926 match, but that the AV interval 930 is greater than the VApacing limit 919. The atrial LRI is temporarily adjusted by apredetermined maximum step size of 100 ms in this example to alengthened AA escape interval 932 of 1100 ms. The RA pacemaker controlmodule 206 determines if the actual AV interval 934 has changed inresponse to the change in the AA escape interval 932. Since the actualAV escape interval 934 shortened by 100 ms (from 400 ms to 300 ms),intrinsic AV conduction is not intact. The FF ventricular event rate isindependent of changes in the atrial rate and is likely a ventricularpaced rate.

If the actual AV interval 934 had stayed unchanged at 400 ms, the longAV intervals would be attributed to slow intrinsic conduction and theatrial LRI of 1000 ms would be restored without further adjustment.Since AV conduction is not present, however, based on the change in theactual AV interval 300 in response to a change in the AA escape interval932, and the ventricular rate is remaining stable at the ventricularpaced rate of 60 bpm, the atrial pacing pulse timing can be adjusted torestore the target AV interval.

On the next atrial pacing cycle, the AA escape interval 936 is againlengthened from the atrial LRI 922 by the maximum step size of 100 ms todeliver the next atrial pacing pulse at another 1100 ms AA interval 936.The actual AV interval 938 is reduced from the original 400 ms AVinterval 930 to 200 ms by stepping out the atrial pacing pulses 100 mson each of two consecutive cycles. Since the target AV interval is 150ms in this example, the next AA escape interval 940 is lengthened only50 ms from the atrial LRI 922 of 1000 ms to an AA escape interval of1050 ms. The next actual AV interval 942 is thereby reduced to 150 ms,the target AV interval. Since the target AV interval has been restored,the atrial LRI 944 can be resumed. Atrial pacing continues at a rate(1,000 ms AA interval) that matches the FF ventricular event rate withthe atrial pacing pulses arriving at the target AV interval 942 ahead ofthe FF ventricular events (FFV).

In the example shown, the actual AV interval 930 is initially detectedto be longer than the target AV interval 942. In other examples, theactual AV interval may initially be detected to be shorter than thetarget AV interval. If the atrial rate and FF ventricular event rate areequal, but the actual AV interval is too long or too short and AVconduction block is verified, the AA escape interval can be lengthenedor shortened, respectively, for one or more cycles as demonstrated inFIG. 11 to change the timing of the atrial pacing pulses relative to theFF ventricular event to bring the atrial pacing pulses to a target AVinterval. It is noted that when the AA escape interval is lengthened, anintrinsic atrial event could occur. As such, a maximum step size may beused to maintain control of the atrial rate by pacing.

In some examples, AV interval correction by AA escape intervaladjustment is performed only when the actual AV interval deviates fromthe target AV interval by a threshold amount, e.g., more than 30 ms. Inother examples, the AA escape interval could be adjusted on abeat-by-beat basis to shorten or lengthen an actual AV interval insmaller steps within a range of the target AV interval.

FIG. 12 is a flow chart 350 of a method for controlling ventricularpacing pulses by RV pacemaker 14 according to one example. RV pacemaker14 may or may not be configured to sense FF atrial events. The methodsdescribed above performed by RA pacemaker 12 achieve coordinated atrialand ventricular pacing by the two separate intracardiac pacemakerswithout requiring RV pacemaker 14 to sense atrial events. In patientshaving intermittent AV block, it is desirable to minimize ventricularpacing in the presence of normal AV conduction. The RV pacemaker 14 maytherefore be configured to provide only back-up pacing whenintrinsically conducted ventricular events are sensed and periodicallytest for a return of AV conduction during episodes of sustainedventricular pacing.

At block 352, a ventricular LRI is established, which may be based on aphysiological sensor signal indicative of the metabolic need of thepatient. An escape interval timer included in the RV pacemaker controlmodule is set to a W escape interval equal to the established LRI.

A conduction check timer is started at block 353. Expiration of theconduction check timer will cause the RV pacemaker 14 to perform an AVconduction check after a period of sustained ventricular pacing toensure that the ventricular pacing is not masking a return of AVconduction. At block 354, the ventricular pacing escape interval is setto a VV interval equal to the established LRI. The ventricular LRI maybe adjusted over time based on a sensor-indicated rate when rateresponsive pacing is enabled. The ventricular LRI may be adjustedbetween a programmed base pacing rate interval and an intervalcorresponding to a maximum ventricular pacing rate.

If a threshold number of intrinsic ventricular events, e.g., at leastone intrinsic event or at least two consecutive intrinsic events, aresensed by the sensing module 204 of RV pacemaker 14 at block 356, thecontrol module 206 of RV pacemaker 14 sets the ventricular pacing escapeinterval to a back-up pacing interval at block 357.

The threshold number of intrinsic sensed events is evidence of normalintrinsic AV conduction. Ventricular pacing at the established LRI isinhibited and only back-up ventricular pacing is provided when AVconduction is intact. The back-up pacing interval may be the base pacingrate interval or another interval longer than an established ventricularLRI. The back-up pacing interval may be set to be a fixed intervallonger than the base rate interval, a fixed interval longer than asensor indicated rate interval, or a rate-dependent interval longer thanthe sensor-indicated rate interval. Back-up ventricular pacing isprovided at the back-up interval if AV conduction block returns.

If one or more ventricular pacing pulses are delivered at the back-uppacing interval, as determined at block 360, AV block may be present.The process returns to block 352 to re-establish the ventricular LRIinterval and restart the conduction check timer at block 353. Theventricular pacing escape interval is set to the LRI at block 354. Ifventricular sensing does not occur at block 356, ventricular pacing atthe LRI continues until sensing of intrinsic ventricular events (i.e.,R-waves) occurs or the conduction check timer expires, as determined atblock 358.

If the ventricular pacing rate is less than a threshold rate, asdetermined at block 362, a conduction check is performed at block 364.If the pacing rate exceeds a conduction check threshold rate, theconduction check is not performed or is performed but after the pacingrate drops below the conduction check threshold rate. The threshold ratecompared to the ventricular pacing rate at block 362 may be less than100 bpm, e.g., 80 bpm, so that a conduction check is performed when apatient is at rest or relatively low activity.

If the pacing rate is less than the threshold rate, the conduction checkmay be performed by setting the ventricular pacing escape interval to aconduction check interval that is longer than the current LRI at block364. The conduction check interval may be set for one or more pacingcycles, for example up to five pacing cycles. The RV pacemaker 14determines if a ventricular sense event occurs during the conductioncheck escape interval at block 366. If sensing does not occur, the RVpacemaker 14 continues pacing at the ventricular LRI. If an intrinsicevent is sensed, i.e., an R-wave is sensed during the conduction checkescape interval, the RV pacemaker 14 returns to block 357 to set theventricular pacing escape interval to a back-up pacing interval. In thisway, the RV pacemaker 14 is not required to sense atrial events in orderto control ventricular pacing in a manner that inhibits ventricularpacing when AV conduction is intact and enables ventricular pacing whenAV conduction block is present.

Thus, various examples of an implantable medical device system fordelivering coordinated atrial and ventricular pacing using separateintracardiac pacemakers have been described according to illustrativeembodiments. However, various modifications may be made to the describedembodiments without departing from the scope of the following claims.

The invention claimed is:
 1. An implantable cardiac pacing system,comprising: a first pacemaker implantable in a first chamber of a heartof a patient and comprising: a first sensing module configured toreceive a cardiac electrical signal and sense near field events in thefirst chamber from the cardiac electrical signal, a first pulsegenerator configured to generate and deliver pacing pulses to the firstchamber via a pair of electrodes, and a first control module coupled tothe first sensing module and the first pulse generator and configuredto: establish a lower rate interval to control a rate of delivery of thepacing pulses, schedule a first pacing pulse to be delivered by thefirst pulse generator by starting a first pacing escape interval equalto the lower rate interval, withhold the scheduled first pacing pulse inresponse to the first sensing module sensing a near-field event duringthe first pacing escape interval, and schedule a next pacing pulse to bedelivered at the lower rate interval from a time that the first pacingescape interval is scheduled to expire.
 2. The system of claim 1,wherein the first control module is configured to schedule the nextpacing pulse by: starting a second pacing escape interval at a time thatthe near-field event is sensed by the sensing module during the firstpacing escape interval, the second pacing escape interval comprising asum of the lower rate interval and an unexpired portion of the firstpacing escape interval at the time that the near-field event is sensedduring the first pacing escape interval.
 3. The system of claim 1,wherein the first control module is configured to schedule the nextpacing pulse by: allowing the first pacing escape interval to expire;and starting a second pacing escape interval equal to the lower rateinterval upon expiration of the first pacing escape interval.
 4. Thesystem of claim 3, wherein the first control module is furtherconfigured to: withhold the next pacing pulse at an expiration of thesecond pacing escape interval in response to the first sensing modulesensing a near-field event during the second pacing escape interval; andstart a third pacing escape interval equal to the lower rate intervalupon expiration of the second pacing interval.
 5. The system of claim 1,wherein: the first sensing module is further configured to sense farfield events, the far-field event occurring in a second chamber of theheart different than the first chamber; and the first control modulefurther configured to schedule the next pacing pulse by starting asecond pacing escape interval equal to the lower rate interval less atarget inter-chamber interval in response to the first sensing modulesensing a far field event during the first escape interval.
 6. Thesystem of claim 5, wherein the first control module is furtherconfigured to: withhold the next pacing pulse scheduled to be deliveredat an expiration of the second pacing escape interval in response to thefirst sensing module sensing a near-field event at a time during thesecond pacing escape interval; and schedule a third pacing pulse to bedelivered to the first chamber at the lower rate interval from theexpiration of the second pacing escape interval.
 7. The system of claim6 wherein the first control module is configured to schedule the thirdpacing pulse by: starting a third pacing escape interval in response tothe first sensing module sensing the near-field event at a time duringthe second pacing escape interval, the third pacing escape intervalcomprising a sum of the lower rate interval and an unexpired time of thesecond pacing escape interval remaining at the time that the near-fieldevent was sensed during the second pacing escape interval.
 8. The systemof claim 6 wherein the first control module is configured to schedulethe third pacing pulse by: allowing the second pacing escape interval toexpire; and starting a third pacing escape interval equal to the lowerrate interval upon expiration of the second pacing escape interval. 9.The system of claim 5, wherein the first control module is furtherconfigured to: determine a time interval from a preceding event in thefirst chamber to the sensed far-field event; compare the determined timeinterval to a time limit; and start the second pacing escape intervalonly if the determined time interval is within the time limit.
 10. Thesystem of claim 9, wherein the first control module is furtherconfigured to: allow the first pacing escape interval to expire inresponse to the determined time interval being outside the time limit;withhold the first scheduled pacing pulse upon expiration of the firstpacing escape interval; and start a third pacing escape interval equalto the lower rate interval upon expiration of the first pacing escapeinterval.
 11. The system of claim 5, wherein the first control module isfurther configured to: allow the second pacing escape interval to expirein response to the sensing module sensing a next far-field event duringthe second pacing escape interval; withhold a pacing pulse scheduled tooccur at the expiration of the second pacing escape interval; and starta third pacing escape interval equal to the lower rate interval at theexpiration of the second pacing escape interval.
 12. The system of claim5, further comprising an acoustical sensor configured to produce asignal comprising heart sounds signals; the first sensing moduleconfigured to sense the far field ventricular events from the acousticalsensor signal.
 13. The system of claim 1, further comprising: a secondpacemaker implantable in a second chamber of the heart of the patientand comprising: a second sensing module configured to receive a secondcardiac electrical signal and sense second chamber near field eventsfrom the second cardiac electrical signal, the second chamber near fieldevents occurring in the second chamber, a second pulse generatorconfigured to generate and deliver pacing pulses to the second chambervia a second pair of electrodes, and a second control module configuredto: establish a second lower rate interval for controlling a rate of thepacing pulses delivered to the second chamber; set a second chamberpacing escape interval to a test interval longer than the second lowerrate interval in response to the second sensing module sensing a secondchamber near-field event during at least one second lower rate interval;set the second chamber pacing escape interval to a back-up pacinginterval longer than the second lower rate interval in response to thesecond sensing module sensing a second chamber near-field event duringthe test interval, and set the second chamber pacing escape intervalequal to the second lower rate interval in response to at least oneback-up pacing interval expiring without the second sensing modulesensing a second chamber near-field event during the at least oneback-up pacing interval.
 14. The system of claim 1, wherein the firstintracardiac pacemaker comprises a housing enclosing the first sensingmodule, the first pulse generator, and the first control module, thepair of electrodes carried by the housing.
 15. A method, comprising:sensing near field events in a first chamber of a patient's heart from acardiac electrical signal received by a first sensing module of a firstpacemaker implantable in the first chamber; establishing by the firstpacemaker a lower rate interval to control a rate of delivery of pacingpulses; scheduling a first pacing pulse by starting a first pacingescape interval set equal to the lower rate interval; withholding thescheduled first pacing pulse in response to sensing a near-field eventduring the first pacing escape interval; and scheduling a next pacingpulse to be delivered at the lower rate interval from a time that thefirst pacing escape interval is scheduled to expire.
 16. The method ofclaim 15, further comprising: starting a second pacing escape intervalat a time that the near-field event is sensed by the sensing moduleduring the first pacing escape interval, the second pacing escapeinterval comprising a sum of the lower rate interval and an unexpiredportion of the first pacing escape interval at the time that thenear-field event is sensed during the first pacing escape interval. 17.The method of claim 15, wherein scheduling the next pacing pulsecomprises: allowing the first pacing escape interval to expire; andstarting a second pacing escape interval equal to the lower rateinterval upon expiration of the first pacing escape interval.
 18. Themethod of claim 17, further comprising: withholding the next pacingpulse at an expiration of the second pacing escape interval in responseto the first sensing module sensing a near-field event during the secondpacing escape interval; and starting a third pacing escape intervalequal to the lower rate interval upon expiration of the second pacinginterval.
 19. The method of claim 15, further comprising: sensing farfield events by the first pacemaker, the far field events occurring in asecond chamber of the heart different than the first chamber; andscheduling the next pacing pulse by starting a second pacing escapeinterval equal to the lower rate interval less a target inter-chamberinterval in response to the first sensing module sensing a far fieldevent during the first escape interval.
 20. The method of claim 19,further comprising: withholding the next pacing pulse scheduled to bedelivered at an expiration of the second pacing escape interval inresponse to sensing a near-field event at a time during the secondpacing escape interval; and scheduling a third pacing pulse to bedelivered to the first chamber at the lower rate interval from theexpiration of the second pacing escape interval.
 21. The method of claim20, wherein scheduling the third pacing pulse comprises: starting athird pacing escape interval in response to sensing the near-field eventat a time during the second pacing escape interval, the third pacingescape interval comprising a sum of the lower rate interval and anunexpired time of the second pacing escape interval remaining at thetime that the near-field event was sensed during the second pacingescape interval.
 22. The method of claim 20, wherein scheduling thethird pacing pulse comprises: allowing the second pacing escape intervalto expire; and starting a third pacing escape interval equal to thelower rate interval upon expiration of the second pacing escapeinterval.
 23. The method of claim 19, further comprising: determining atime interval from a preceding event in the first chamber to the sensedfar-field event; comparing the determined time interval to a time limit;and starting the second pacing escape interval only if the determinedtime interval is within the time limit.
 24. The method of claim 23,further comprising: allowing the first pacing escape interval to expirein response to the determined time interval being outside the timelimit; withholding the first scheduled pacing pulse upon expiration ofthe first pacing escape interval; and starting a third pacing escapeinterval equal to the lower rate interval upon expiration of the firstpacing escape interval.
 25. The method of claim 19, further comprising:allowing the second pacing escape interval to expire in response to thesensing module sensing a next far-field event during the second pacingescape interval; withholding a pacing pulse scheduled to occur at theexpiration of the second pacing escape interval; and starting a thirdpacing escape interval equal to the lower rate interval at theexpiration of the second pacing escape interval.
 26. The method of claim15, further comprising: sensing second chamber near field events by asecond sensing module of a second pacemaker implantable in a secondchamber of the patient's heart different than the first chamber, thesecond chamber near field events occurring in the second chamber,establishing by the second pacemaker a second lower rate interval forcontrolling a rate of the pacing pulses delivered to the second chamber;setting by the second pacemaker a second chamber pacing escape intervalto a test interval longer than the second lower rate interval inresponse to the second sensing module sensing a second chambernear-field event during at least one second lower rate interval; settingthe second chamber pacing escape interval to a back-up pacing intervallonger than the second lower rate interval in response to the secondsensing module sensing a second chamber near-field event during the testinterval, and setting the second chamber pacing escape interval equal tothe second lower rate interval in response to at least one back-uppacing interval expiring without the second sensing module sensing asecond chamber near-field event during the at least one back-up pacinginterval.
 27. The method of claim 19, further comprising sensing the farfield ventricular events from an acoustical sensor signal.
 28. Anon-transitory, computer-readable medium comprising a set ofinstructions which, when executed by a control module of a pacemakerimplantable in a chamber of a patient's heart, cause the pacemaker to:sense near field events from a cardiac electrical signal; establish alower rate interval to control a rate of delivery of pacing pulses;schedule a first pacing pulse by starting a pacing escape interval setequal to the lower rate interval; withholding the scheduled first pacingpulse in response to sensing a near-field event during the pacing escapeinterval; and scheduling a next pacing pulse to be delivered at thelower rate interval from a time that the pacing escape interval isscheduled to expire.